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Frame challenge:

You can’t, at least with the limitations imposed

You are asking for too much, some of your points contradict each other or ask for something that's far beyond what a sword could do:

Able to cut through steel, i.e. through a sword raised in defense or plate armour.

Firstly, asking to cut through a steel sword with another sword is hard enough. The problem is is if you raised your sword and I struck it with my own, your sword is going to flex and move away. Your arms may also move to absorb the blow, further reducing the force on the sword.

Second, that's not how you fight with swords anyway. It's not as simple as “swing the blade as hard as you can and cut the person”, it has more finesse than that. I won’t bore you with all the fine details about sword fighting, but I will give you the basic concept of “overly strong strikes”. Basically, if your opponent is swinging with a lot of power (as they would be if they were trying to cut through your sword), it is futile to try and match their strength with your block, if they hit the tip-end of your sword, it will move out the way anyway and you’ll be cut. If however you instead let them knock your blade aside and you step out the way, you can redirect your sword with the momentum they gave you and strike them instead, using their strength against them.

Made like some kind of saber for cutting (based of off the nodachi, zanbato, or dandao)

None of those swords you listed are ideal for a duel between sword fighters, as they don’t have large enough guards. A strike is more likely to hit you as you have very little protection, cross guards, when used correctly, protect your entire body, not just your hands. I would recommend something like the Kriegsmesser, since the blade has similarities to those swords you mentioned but the cross guard is far larger and there is a “nail” that sticks of to one side, offering even more protection.

Rarely needs to be sharpened

It will need to be sharpened often, there is no two ways about it. You are striking a fine edge against solid steel, you will need to sharpen it, if nothing else but to remove the damage to the edge of the blade. Also, even the air causes steel swords to dull and require sharpening. In short, your swords will need to be sharpened fairly regularly.

isn't affected by water or air exposure
Hard to break, snaps back into shape

No idea what “snaps back into shape means”, if you’re referring to the damage of a sword being repaired easily, I would just like to point out that it will always be weaker than before it was damaged.

However, steel swords could cut through iron or bronze swords

All this being said, it is still possible to achieve something similar to what you want, though not against steel. Both iron and bronze swords can be cut through with a steel one, there are a number of accounts of this.

If you were to change what swords everyone else was wielding, your answer becomes “the monks use steel swords”. Steel smithing was a “mysterious” process during the Bronze Age, it could cut through iron or bronze swords, could easily be made into any sword design you wanted, wouldn’t need sharpening as much as iron or bronze and the process of making steel stainless makes it resistant to water and air corrosion (though, to clarify, you’d want stainless spring steel or stainless tool steel, not the stainless steel cutlery, cheap pocket knives or training swords are made out of).

Here are some videos which talk about steel swords and why steel was used for swords historically:

• The oldest STEEL sword in the world, Vered Jericho sword of Ancient Israel RECONSTRUCTED


• Super cutting swords, are they needed?


• The riddle of steel: How people made it by accident for millennia

You won't be able to cut a sword as if it were paper, cloth or a bamboo, not with your constraints. Mainly because metal blades are not made of fiber, so even for a medieval sword you would need a laser for a proper cut.

What you can do is make the enemy's blade snap. Blades will snap if you bang them hard enough against hard objects. To that end, ancient chinese developed the bagua zhang dao:

That thing does not have a sharp edge, and that is by design.

Modern ones for martial arts training weight up to 2kg (~4lb), but my fencing instructors told me ancient ones could weight up to 10x 4x that. You wouldn't swing those blades like a regular one; you'd have to keep it in constant motion, switching arms and using your torso as a counterweight. The point of using that weapon in battle was that if your opponent wielded a regular 0.7 to 1.5 kg sword, blocking your blade with their own would mean their weapon would break.

And if they were using armor, taking a blow from a 4-8 kg piece of metal with a very thin profile may be lethal. The armor will bend and press your body into an unnatural shape, breaking bone and tearing ligaments. If the armor is brittle, it may not stand the reshaping and tear open.

Edit: After talking to one of my instructors, he said 10x was a gross exaggeration. Most bagua sabers wouldn't be more than 3 or 4 kg heavy, and only some (maybe ceremonial) ones would reach 8kg. Still, we broke a lot of swords when we sparred with smaller, lighter blades; it just happens that in ancient times, a blade would hardly last more than one or two battles, depending on the kind of blade. I know for sure that a 1kg sharp one parrying a 4kg blunt one will probably be damaged beyond usefulness after not too many direct hits.

Another edit: I learned some moves that I used to practice with a wooden one (which was on the light end of the spectrum, probably 1.5 to 2kg). They look like this, but are not exactly the same. The most similar part is the vertical swing followed by a low swing at 0:20.

Ceramic swords

We have ceramic knives today, much harder than steel (8.5 on the Mohs scale of mineral hardness, compared to 4.5 for normal steel and 7.5 to 8 for hardened steel). They keep their edges far longer than steel blades and never rust or degrade, even in very harsh conditions. They are also resistant to acid.

Ceramic blades are, however, more brittle than steel, so real-life ceramic swords will not live up to your last requirement. Your monks may have solved that problem through a secret process. In Miyazaki's Nausicaä manga (pucture) and anime, the heroine wields a ceramic sword that slices through steel blades, so there is a fictional precedence.

The monks are master swordsmen.

through: moving in one side and out of the other side of (an opening,
channel, or location).

The monks know swordsmanship. To people who fight them (or actually people who have talked to people who have seen the monks fight) it seems as if armor and swords pose no barrier to the attacks of the monks. Their swords cut right through. If my offense comes through your defense, possibly I have shattered your defense. Or possibly, with my sweet monkish skills, I have sidestepped and bypassed your defenses and come through that way. As @Liam Morris points out, sword fighting is very much about skill; it is not just chopping down a tree (which also requires a modicum of skill).

The swords of the monks cut through steel defenses not because the monks are alien material scientists, but because they have such sweet martial arts skills.

The monks do not discourage the "magic sword" idea. They actually have some wild looking glass swords and other strange relics displayed in obvious places around the monastery. If you ask one of the monks about these swords, he will nod and smile.

Your problem is physics, not metallurgy. Steel is tough, even if your sword is perfectly rigid and takes no damage during a hit, you'll push the armor wearer back.

To puncture the armor, you need a lot of force concentrated in a very small spot, but then armor thickness of a plate mail is laid out to resist such stabs.

A much sharper material won't help you much, since it won't improve the muscle power of the fighter. Lighter materials also don't help, since you actually want the weight of the blade to transfer momentum, and tougher materials also don't help, since that makes your blade prone to shattering.

You have another problem:

Suppose you had an infinitely sharp edge with no thickness: You still need to separate the molecules of steel from each other. This imposes a certain minimum energy per area cut.

Probably you can get a first order approximation to this energy by looking at the power used by metal shaving equipment.

Suppose that it turns out to be 1000 joules per cm2 (Number picked out of my a... air.)

If your weapon weighs a 2kg, and you get it moving at 10m/s it has $frac1 2 mv^2 = 100 J$ And so it would cut 1/10 of a square cm of steel -- roughly a 1 mm deep cut 2 cm long, with tapers at the ends.

I have zero faith in that 1000J/cm2 figure.

A different framing challenge: why specifically swords?

@LiamMorris has given a really good framing challenge on whether steel swords could be cut so I want to give one on using a sword at all (or as the saying goes: "perception is reality")

In modern times, Unicorns invoke an image of a white horse with that spiral horn but early on people thought these were real creatures. It has been theorized that the myth and reality of unicorns came due to far-away or obscured views of giraffes and rhinos. Indeed, the term unicorn was also used to describe rhinos in Europe and in Japan the kirin (Japanese for Unicorn) that was presented to the Emperor turned out to be a giraffe.

What does this have to do with swords? Nothing except that:

• appearances can be deceiving


• second hand accounts (esp. if based on a 1st hand account that was given by a person in shock) are not the most reliable


• people who write histories (especially ancient ones) are not always 100% accurate

Use a club

So given the other referenced framing challenge and the other answers based on cutting steel: if we try to "cut" through a a steel sword it's just not going to work. However, if we have a weapon (a club really) that looks like a sword it at least has a chance to actually break (not cut) the sword. If an onlooker is:

• Fighting his own battle and in a high adrenal (fight or flight) state & only catching glimpses


• Staying far back to avoid getting drawn into the battle


• Hiding under some kind of cover or concealment which can obstruct the view

and then sees our monk use his club to break the blade at all (even if just bends the blade or break means it knocks the blade from the tsuka or handle). There is a good chance he would think "Holy *@!*, did that guy just snap that other guy's sword in two with his own?!". Add a few re-tellings and you have the "Legend of the Monk's blade which cuts through steel".

Japanese "Sword Breakers"

If there is a Japanese weapon, that already has a legend and nickname attached to them that relates to this it is the "sword breaker", the jitte or Jutte:.

With the above you could strike a katana₁ and cause its blade to break or bend if it was cheaply made, had a defect, or was not taken care of properly (either damaged due to lack of skill or something like the tsuba not properly being attached after a cleaning for a story reason). History is fuzzy on if these were actually used to break swords but I've personally knocked the blade from the tsuba on a cheap (<$100) katana with my baton so I know it is at least possible that this had happened before₂.

Or maybe these monks had contact with Europeans? The Bar Mace

Another excellent example, thank you Liam, is the Bar Mace (picture from "European Weapons And Armour" by Ewart Oakeshott):

Ewart describes this one as an Italian example from the 13th century which would also add to the mystery - only the monks have a weapon like this. Beyond the fact that someone seeing it from the side in action could be all like "What kinda crazy sword is that?!" due to edges, profile, and usage.

So all you would then need is someone to start telling the story of how they saw these monks fight these bandits armed with katanas (which are actually very cheap and/or poorly maintained but the peasant wouldn't be able to tell that), where one of them broke the bandits leader's sword into two pieces when he struck it with his own "sword". Soon the "telephone effect" will start and its turned into how he "cut the sword in two with his magic sword".

_{1: Note, with the hook variety: you would not catch the blade with the hook, rather you would block with the "blade" then hook the katana blade or, more likely and safer, catch the attacker's arm/wrist/cloths with the hook to apply control techniques. This is very likely the real reason they were named "sword breakers" they "broke" the sword's attack but the legend was attached and its basis in reality contested to this day.}

_{2: It didn't even break the blade just knocked it from out of the tsuka (handle) due to it only being attached with a very small tang. Still heard people say the sword (blade) "broke in two" despite this fact.}

As far as I know no sword was really able to cut through metal armor. It's common in fiction, but probably even when shown on medieval drawings: it was exaggeration to show the might of a king/famous knight and so on.
Most of the cuts found on skeletons by archaeologists on old battlefield/post-battlefield mass graves are found on hands and legs, since they were often not covered by armor.

You may find some YouTube videos showing cuts going through the armor, but they are executed on very thin, low quality objects, being held stationary.
In a real fight you would require so much force to cut through something that hard, that even if you had enough strength and sword that wouldn't break in the process, you would firstly push your opponent, since they wouldn't stand stationary, waiting for the sword to go through, force of the hit would push them (and probably also slide off from the initial point of impact), instead of pushing blade through the layers of steel.

Swords breaking on each other were a thing, but doing so on purpose not so much. Especially that again: if something is pushing at your blade strong enough to break it, your hands will bend or break first.

So: I don't think you can find any historical precedent or very down to earth justification of something like that occurring. You will need some magical mambo-jumbo, but it probably should contain a mix of superhuman speed, so cut goes through faster than body/sword is able to move out of the way, that would force sword to go into the armor instead of pushing target away and amazingly hard steel (that would also make it preserve its sharpness), so it won't break or shatter in the process. And, because the blade slides off the target whenever it can (seeking for the path of least resistance), fighter would fave to land cuts on ideal angles, so the blade won't end up laying flat/sliding off.

Let us take an example perfect edge. An edge is a mathematical line, at the intersection of two planes or curves. The perfect edge is perfectly straight and perfectly thin, that is, with zero width. Any deviations from straightness or thinness make it worse at cutting.

An atom is the thinnest width that a physical edge made of atoms can have. That's at the atomic scale. A midieval sword might be a million atoms across or worse, with a broad, blunt edge pressing and scraping across whatever you want cut. Already we are losing cutting ability.

The perfect edge is made of perfectly strong material that never wears away (chips, burrs, or cracks). In the real world, it must be made of atoms, which have bonds of limited strength to their neighbors. When that bond's strength is overcome, the atom is ripped away or shoved away from the line, distorting the straight edge. Chips, burrs, and cracks appear in the line. We are losing more cutting ability.

If you want to cut through steel armor (or another sword), you would need a very thin, unbreakable line for the edge. In science fiction, Larry Niven's Variable Sword is appropriate. The Variable Sword is a handle, a little glowing button to show the end, and between them an invisibly thin unbreakable stiff thread. It wielder cuts through most objects without effort.

A real-world sword is made of atoms, which support each other and form the 99.9% of the sword that isn't edge. (99.999999...) The atoms must be bonded together so that the sword doesn't wear away quickly. But materials have several different measures of "strength" : compressive strength, tensile strength, hardness, how far the shape can bend and still return to its original shape, etc. The melting point is important, too, as well as characteristics as the solid heats up, because a hot sword is easier to bend.

Midieval crafters don't have the materials science to define good steel, let alone create good steel, let alone create a large amount of good steel repeatedly. If they somehow got their hands on an excellent non-steel material, they would have a very hard time putting into the shape of a sword, let alone putting an edge on it.

Let's imagine they isolated ten pounds of tungsten (it's almost thrice the density of iron). They would need the materials science to make a ceramic mold that could withstand its tremendous melting temperature. That makes a sword-shaped object with a blunt edge, and they have to put tremendous amounts of time and effort into sharpening it. And still, its edge would be a million atoms across, and still, using it blunts the edge.

Other very hard elemental metals have similar problems. Some of them are quite toxic as well.

Let's imagine they acquired a sword that is a single crystal of diamond. Nice and light, but it will chip and shatter easily. Also, the carbon gets absorbed into iron and steel it presses into, so the edge will quickly be ruined.

Long story short - steel is used for swords because it's the best compromise known of all the problems known. A society needs science to be able to define better materials. Better materials are frequently just more-precisely-understood steel with vanadium, tungsten, molybdenum, etc. And it's still not going to cut through armor or other swords without blunting.

Steel is not a simple thing, it can be engineered to have a range of different properties, and in fact many of the tools used to work with steel are made from different kinds of steel, e.g. tool steels https://en.wikipedia.org/wiki/Tool_steel and high speed steels https://en.wikipedia.org/wiki/High-speed_steel

To make plate armor, you'd instead want a relatively malleable steel that could be beaten into shape. Thus a sword (say a Japanese katana) made from a hard & tough steel could cut the armor, if wielded with sufficient force. Likewise, if the other people's swords are made of such steel (because they don't know the secrets of the "magic" steel), then it might well cut a sword used for parrying. Though that would really be inferior technique...

Your monks, through going into battle with a pre-wound motor, are now able to make their blades vibrate at the right frequency to have the motion do the cutting, not whacking the blade (it is now a very fast saw). If they are advanced scientifically, they can make a motor with an uneven weight on the end, powered by a rotational spring (or spool of string for short engagements).

Some drawbacks:

1. It will not cut through like nothing is there. You have created a saw, not a lightsaber.




2. Vibrating motion, for an extended period of time, will number their hands, or at least be uncomfortable. They may be able to withstand this through monk discipline.




3. It would not work for extended engagements where many strikes are needed, as your motor would run out of preload.




4. The blades would have to be thinner than normal blades. This would make them more brittle, unless your monks are better at treating steel.

I don't know how realistic you want this to be, but maybe these monks offer up a special prayer to Combat God, and use the power granted to vibrate swords for an extended period of time. Although, at that point, you might as well have them pray for a lightsaber.

I don't know how SuperTech you wanna go, but if you want something lightsaber-ish but not a lightsaber, you could have a very thin ceramic heating element, heated up to around 10eV (115,000 K), not a sword, but a very very Hot .... Rod.

OK, so your HotRod emits blackbody radiation at a peak of several eV (hard i.e. ionizing UV and soft x-ray) so it doesn't glow in the visible spectrum, but it will ionize and thermally excite air out to some distance. Depending on how hot you want the HotRod to be, this glow could be confined to a few mm from the rod (lightsaber-ish) or it could be spread around a 10 m radius sphere (The monk appears cloaked in a nimbus of strange and whirling lights.) The Monk weilding it is definitely going to eat some Rads, but 1) Monks are already head-shaven, 2) Monks might have a vow of chastity (don't start the HotRod next to yer jimmies!!!) and 3) Monks might have some advanced medical tech or secret herbal tea that heals radiation. A captured HotRod is effectively a cursed item to a non-monk, who promptly contracts a strange wasting disease upon playing with his monk-toy....

OK, so what happens to a struck sword? Assume a straight bladed sword with a diamond shaped cross section 3cm x ½cm, and the strike affects 1.33 cm of blade lenghth (this works out to 1 cc) with the steel blade having a density of 10 g/cc (high for steel, which is ~8), a thermal conductivity of 50 W/mK and a specific heat capacity of 500 J/kgK (All of these are reasonable numbers, Thanks Google!). When the 1 cc of steel blade comes into contact with the HotRod at 115,000K, it will suck heat at a rate that will change it's temperature by 34,200 K/s. After 50 ms of contact (basically instantaneous as far as human reflexes are concerned), the assumed volume of steel blade in contact with the HotRod will heat up by 1710K, from a starting point of, presumably, 290K (room Temp), this means that the portion of the blade struck will be 2000K after a twentieth of a second, and steel melts at 1650K. So your HotRod just turned a portion of steel blade into slag, with 1 solid portion falling away and the other still attached to the handle, it's mideival owner looking like a sap...

There are some wonderful answers here, mostly explaining that steel sword on steel sword is a tough nut to crack because of good ol' physics and metallurgy. There is another way to do it though....we can cheat with the power of narrative convenience and swipe some ideas from old sci-fi stories.

Your Monks will, of course, come from Shambala or some other mystical mountain fastness. It always has perfect weather, just the right kind to make things grow, to ward off violent storms, and so on. This is because it is the home to the Legendary Quantum Weather Butterfly (Thank you Sir Terry Pratchett) This butterfly can alter the weather by flapping it's wings this way or that way because the edge of it's wings is fractal, and therefore infinitely long. The good weather is only the bonus though. The real key is the silk produced by the caterpillars that eventually become the mature Quantum Weather Butterfly.

The interesting thing about this silk is that it is not just really fine, but that each strand is, in fact, mono-molecular. The real skill these monks have is not in making steel (they aren't slouches though), but in being able to handle the raw silk and using it effectively.

What the monks do is craft a mediocre steel sword with blunt edges. Then they wind the silk around the tip and the handguard. They then run a single strand from tip to guard, getting it as taut as they can maybe as much as 1mm away from the actual blunt edge of the blade. Given that the strand is one continuous molecule, it will have the same strength as the bonds between it's individual atoms, so the thread itself is nearly indestructible by normal means. Also, since it is only one molecule thick, it will be able to cut through just about anything like a wire through a block of cheese.

Anyone observing these blades in action will be astounded at how they seem to shear through inferior steel weapons and armor. The cut will seem to begin slightly before the blade makes contact with whatever when seek up close.

The second point is that through their martial arts training, they are actually going to be striking the enemy weapons weakest points. The haft of the spear or mace, the handle of the axe or sword, the straps or joints of the armor.

The swords themselves will probably have significant wear after a handful of battles. but will just need to be rewound with fresh silk.

So the monks provide a safe place for the butterflies, and the butterflies give the monks awe inspiring weapons, cool clothes, and the kind of weather where you don't mind not having hair.

You are after the old style Damascus blades made from Wootz steel.

To make a long story short, about 2000 years ago a certain type of steel came out of india known as Damascus steel (not to be confused with modern damascus). This steel was reknown as far stronger, held an edge longer, and handled flexion better. You can read about it on wiki.

https://en.wikipedia.org/wiki/Damascus_steel

It was reknown to be able to cut through other steel, shear stone without blunting and had a recognisable pattern across each blade.

Essentially the art of making it was lost as the blacksmiths died and modern attempts to recreate it have failed. Although in examining the blades modern science has uncovered carbon nanotubes inside them, (carbon nanotubes are aproximately 100x stronger than steel and harder than diamond) which is incredible as carbon nanotubes are a relatively recent scientific discovery.

I feel like you could quite easily incorporate this type of steel into your world and the real world story behind it fits in very well with what you are trying to create. Sorry I dont have more details but a google search should provide them.

The main thing is not to confuse modern damascus steel with the historic version. Modern damascus steel is just pattern welded steel to try to recreate the pattern, and while it can sometimes be stronger than other steel, it does not bear the same properties as the historic version.

Here is another article on it.

http://www.ancientpages.com/2017/04/05/ancient-secrets-damascus-steel-legendary-metal-used-crusaders-warriors/