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The setting is typical low fantasy

Undead rats and fleas, no more modifications needed.

The conflict is a pitched battle on an open field, both sides having time to prepare.

Remember the Night King from Game of Thrones, and just about any other undead overlord. They do well in attrition war, not direct final battles.

Just send some fresh reanimated rats the enemy's way and go home. Come back some five years later and if those guys are anything like medieval Europeans, cities will be mostly empty and people will be digging mass graves everyday.

Edit: Monty Harder suggested using bats instead. They can also spread the plague, and can target specific individuals. A few bats to pick valuable targets might be nice. But fo stick mainly to rats, though, as infecting live rats with the plague will keep the disease self-sustaining for a century.

As far as I see, there are two main variables here - the time spent on the preparation of the unit and the maximum number of units active simultaneously. A slightly less important constraint is the resource one - the amount of dead bodies available for processing. The problem is that without the precise numbers and calculations for all this, the answers will be opinion-based.

The task most resembles the question of gameplay balance for strategy games. That means, there will be multiple optimal creature designs for different strategies (one necromancer will go for zerg rush, while another will invest into heavier and more expensive units with heavier armor).

Now comes my opinion-based part. First I will describe my assumptions. It seems that your necromantic magic doesn't have any predispositions to humanoid shape. Second, the produced units are not extremely smart. I can't understand, how good their coordination is, but most likely it's not good enough to handle projectile weapons. Third, the necromancers themselves are the product of their own time and culture, so they will also be thinking in the terms of the medieval war.

What it comes to, I think, is designing a framework of a combat robot for a medieval battlefield as envisioned by a medieval necromancer, who tries for maximum efficiency. So, a basic 4 to 6 legged shape, low-slung, muscular, having claws or bone blades on all the limbs. They do not need vertical posture or opposable thumbs, being too stupid to be the tool users. I am not sure the creature even needs a head - it's too obvious a target. Better to have an array of sensory organs all around the body for the maximum field of view. The smaller creatures (zerg rush units) are about human mass of lower, extremely fast, designed to shorten the distance and swarm the enemy quickly. They can be used for ambushes, raids, etc.

For open-field battles you would need a heavy cavalry analog - and I think the same design, but upscaled to weight up to a ton, and armoured in bone plate. If the swarmer is something like dog-spider-xenomorph mixture, this is more of a huge buffalo-spider mixture. Heavy, armored, with forward-projecting horns, it will just have an edge over medieval infantry and cavalry.

I'm not sure you would need even heavier units. Maybe something huge and centipede-like for siege scenarios to serve as a living ladder to put your troops on the top of a castle wall.

UPD: I've missed the 'shambling' part in the list of the requirements. If they can't develop high speeds at all, then smaller creature are not cost-effective. They will be too slow for their mass, and if you need to spend a day for each one, and can't just raise zombies by thousands, then the living will be able to dismember them faster then you create them.

I'd say then you would need a mixture of a hussite war wagon and an authomatic thresher - an armoured many-tentacled monstrosity. It would just need to waive all it's heavy (and also armoured) limbs around and move forward slowly, while the opposition tries to find a vulnerable place.

For a standing army, you will probably get the most bang for your buck from simple skeletons/zombies covered in a bone plate armor and wielding bone clubs.

Reasoning:

The strategy here is to take a horde of undead that is presumably already available, and with out significantly reducing their numbers, equip the army with the most cost effective equipment and armor possible. This is because if a necromancer already has a large army, any further improvement to the quality of said army will be a greater force multiplier than just adding more bodies to it.

The choice of equipment is therefore crucial. Our choice is shaped by what will be useful in the widest range of scenarios. Thus we pick the simplest undead shape to manage (skeleton or zombie) and equip them with most cost effective equipment available to us; equipment made out of dead creatures.

The equipment:

I have selected a bone club as the ideal starting weapon. This is because it is useful in a wide range of scenarios. A club can be used against armored opponents more effectively than a slashing or piercing weapon, because concussive force travels through armor. Slashing from bone swords or piercing from bone arrows likely won't penetrate armor however and is not as useful. A club can also be used against unarmored opponents, so this simple weapon is useful in all likely scenarios. The club could be a Femur, which should be amply available in any undead army.

In the event of overrunning a living army, a necromancer might then want to upgrade their troops with maces and other weaponry that can be picked up off the battlefield. However an undead army should get a lot of mileage out of clubs.

For defense I have selected bone plate armor. Bone armor has existed in the real world in the past, so assuming that bones are plentiful and easy to shape for a necromancer (or their bone smiths), it should then be possible to equip an undead army with some crude version of bone armor. Even just loose bones tied together would probably make a big difference in survivability.

Conclusion:

With that, the necromancer now has a basic undead army with decent protection and offensive ability. The army is now more than just a bunch of mindless shambling undead, it is a horde of armed and armored undead. However the real benefit is that we have improved this army's offense and defense at minimal cost. We used materials that were already available and that cost us little. So it should be possible to still have a vast horde of undead, but now they are may times more effective because a simple cavalry charge will not wipe them out.

Looking at this as if I was the general of the necromancers, my first order of business would be to figure out the number of followers I have, the time until the battle, and the skill of my subordinates. I would be smart to make sure the most skilled necromancers were working on the most powerful minions (if they can control larger zombies more effectively they shouldn't waste their time on making one small zombie per day). It is always more effective to have a variety of troop types, as they allow you to counter a wider variety of attacks.

A big flaw often in undead zombies is a lack of long range protection and fire power. Due to the stumbling nature of the creatures, I assume they would not be able to effectively handle projectile weaponry.

All this put together, I think the ideal flesh monster would look something like a human bolder. Layers of flesh around a brain that is the controller, and muscles that can expend and contract the sides of the sphere to increase speed and turn. This turns slow stumbling creatures into fast, maneuverable opponents, and also gives the control center of the creature the maximum protection from projectile weapons. Because the fight is on an open field, little would be in that environment to halt the momentum of these flesh boulders. You could still have lower skilled necromancers raise traditional zombies from the dead to clear the maimed living from the battle field, and provide protection for the casters. The boulders could be filled with what ever human parts can be found, and rigged to explode if they are close to being defeated. Covering the enemy in the corpses of humans would likely create intense discord among enemy troops.

You have two mutually exclusive requirements:

• They are all shambling and not very dexterous.


• The conflict is a pitched battle.

My knee-jerk choice for shape is that of a dog, but "shambling and not very dextrous" kills that idea. IMO, that leaves one, simple shape:

An armless, headless human

Your only hope is to force the army off the field. To disperse them and make their organizational advantage worthless. That army will, given a chance, cut any individual zombie to shreds. This means you need to win by numbers and not by force of arms. Your zombies need:

• Tough hides


• Strong legs


• Good balance


• Low center of gravity

After which all you need do is cause them to "move forward." You're not doing anything complex like, "parry that blow!" which would be very complex when dealing with hundreds or thousands of zombies (which don't sound like they have a tremendous amount of independent thought).

And to be honest, faced with a larger force of "they're going to just walk forward, over us if possible" zombies, would likely cause a lot of warriors to soil their armor.

If we're talking about minions meant to meet enemy force in battle, I vote for fairly large (bear- or elephant-sized) conventional forms with redundant limbs/methods of moving. Possible modifications include extra bone, scales, or grooved hard surfaces for deflecting weapons intending to lop off limbs.

Options not relating to overall body structure will also be helpful. For example, putrefying organs or structures producing disturbing, frightening sounds could lessen the effectiveness of any soldiers that get in close enough to do any damage.

Large size is valuable because it's harder for smaller numbers of living enemies to cut them down and they (through sheer mass) can do a lot of damage without needing precise coordination. Even a shambling step can scatter opponents if the foot and leg are large enough.

Redundant limbs or redundancies in other methods of locomotion are valuable because these troops need to keep moving forward (that's basically all these things can do anyways). These modifications keep a unit from being easily made ineffective and greatly increase the amount of work an opposing army needs to do to win.

Big big big.

https://warosu.org/tg/thread/48439094

Like this guy. 500 corpses, at least. Hugely huge. I suspect something this big might walk like a gorilla on all 4 limbs most of the time.

Too big for one person to control, but you do like the puppeteers do for big puppets: team up. Each limb has a couple of necromancers to control it and the whole thing is a team effort. Plus a monster this big has room for a nice howdah on top so the necromancers can ride into battle.

There is nothing a conventional army has that can stop something like this except maybe a direct hit from a big trebuchet. When your colossi take the field, go get those first.

What you most likely need is some versatility, but mostly superior numbers.

First you need the mob force. These guys should just be easy to make. Most likely unmodified humans. These guys will go down easy, and we will get back to them.

Second you need your set peaces. Go crazy here, stitch 10 cows to an elephant. These things may barely move, and be worthless in combat, but they will scare the crap out of your enemy.

Finally you need your linebackers. This is where you put your most talented necromancers. These guys main job is to not go down. Use humans, or whatever else you have, stitch bone or whatever else you have to armor them. Give them the best weapons, and maybe a few extra arms to hold 2 shields and 2 spears. Maybe extra legs to stay balanced and hold their ground.

Here are the battle tactics. Set up your mob in front. They are cheap, you hopefully outnumber your enemy. Go wide, don't get flanked.

Put your big guys on the sides. Put your archers on them. Their have one job. stop a flanking cavalry charge. Horses can't plow thorough 10 cows, and if you try, have the zombies on top shoot at them, or throw thing, or jump down.

Great so now we are forcing the enemy to charge you head on. Great, they will quickly take out your mob units. Let them. Let them get in deep, then reach your back line. This is where they stop hitting your linebackers. These guys should hold the charge.

Now have your "dead" mob get back up. They rise into the army attacking the line backers. They feel no pain so all the injury they suffered does not matter. The enemy is in total chaos, They are trying to push in at a foe that is just trying to stand ground while things bite and tear at their legs. Have the linebacker elite forces now move forward and kill.

Gather up the fallen to replace anyone damaged, and restock the mob. You are ready for battle once again.

Your biggest threat is an army knowing your tactics. They may try to use their superior mobility to circle your forces. Try fight with defensive terrain behind you. Don't be afraid to retreat. You have the benefit of stamina. Make them chase you trying to flank. You can backup faster than they can move a whole army behind you. Eventually they will have to frontal attack or back off. If they back off, attack at night. That is an easy victory.

Charger: A rhino with a hollowed out chest that can hide secondary units inside. The rhino has two limbs with sharp blades on them to fight things directly surrounding it (give him more sets that will deploy if the original limb is destoyed, or to attack more things) Then add a tentacle or some other limb on the back that lets you launch projectiles.

Next comes the Doom root. a large mass of muscles that serves as a catapult. when the catapults are nolonger needed it will untangle it's muscles and move towards the battle lines and root itself and starts to attack the back rows while the front lines are occupied with your other minions. additionally this could be used as a anti cavalry field, your equivalent of a pike line.
With a smaller variant that uses a number of tentacles to pepper the enemy, maybe could be carried by the rhino as a mounted ranged unit. if it falls off or the mount is killed it uses the sharp protrusions on it's tentacles to mess up the soldiers and make as much damage it can before pulling back.

The collector
a human shaped giant that collects bodies on it's spikes to make a padding out of the bodies(D&D style)

The death caller a large multi-legged creature with bodies hanging off of it. it has long limbs that can pick off important units, like commanders if it gets attacked. (could be harpoons on a long muscle that can pull back quickly)
It serves as a command vehicle as well as a carrier for units. very light top made of bones with robust limbs. (the form would vary, A giant centipede, the aliens from war of the worlds, a high tower or obelisk. The point is the very sight of this is scary)

Cruncher: a mass of bones and muscles (could be like a worm or a beetle) that can dig dirt and stone and compress it into a projectile for the doom root, can also dig out trenches and burrow itself to attack from bellow

Blood rain: Primarily a siege unit. a large gut filled with gases, that floats over an area and covers it in body fluids. blood, stomach acid, bile etc.
Demoralises the units and spreads diseases without leaving a large scale impact to the are, like a full on plague would.

Bone beetle hangs on the chest of the charger and drops off once it plows into the front lines. then jumps at a soldier and tries to chew and slash until he is critically wounded, then lets them cry in pain and jumps to another one. They are hard to kill, because of the bone plates covering it. the exoskeleton is very useful here.

That's just examples to get you going. I'd say the bigger and more versatile the unit is the better it will be for your purposes, because you can make just one per day you will have to make sure they last a long time before going down. I'd say the first two units on the list are very versatile, so those are my best examples. But more important than individual power is the ability to complement the other units on the battlefield, always.

Poison minions.

Disease, sickness, and poison. These are the biggest threats to medieval towns and armies. Also fairly true for more modern day warfare in trenches.

Construct minions that can go an poison water supplies. They just need to quietly go into the wells and latch on, and poison the water.

Open fields of battle, a flash golem is going to be your best bet. 100 necros making 1 golem a day can field a sizable army in a few weeks.

I picture the ideal mass-produced patchwork soldier being a large (seven or eight feet tall), vaguely humanoid thing, with lots of extra muscle. The extra muscle would make for a good damage sponge, with lots of people hacking and slashing at it to little effect.

Its arms would end in concentrated masses of flesh, with bone shards sticking out every which-way, and some bone plating for good measure. @tyler s. Loeper made a good point about blunt force damage against armor, and these flesh-and-bone clubs would be excellent for that. It would have an extra pair of legs, to stabilize it while swinging its meat clubs, and to keep it from falling over.

This would be an easy-to-produce minion, not much more complicated than slapping some extra meat and limbs on a human corpse. They wouldn’t have to think very hard, with their battle tactics not needing to be more complicated than “I have big things, swing them at people.” Plus, if we’re not worried about decreasing their mobility (since they’re already clumsy, shambling puppets), you could add some bone plates to keep some especially tenacious soldier from lopping off one of its oh-so-precious arms.

I'd suggest one possibility is something tiny, like mites. Send them inside the enemy to feast on their vital organs. If you are going to only have these, avoid being (the necromancers) on the field of battle yourself, as a rush of mites won't protect you from the cavalry charge...

There is no "best undead form."

Your necromancers need a strategy, and they need to tune their minions to that strategy. Their strategy should be defined by their goals. For example, if they want everyone dead, they should focus on plagues and destroying food sources. If they want a negotiated settlement with concessions, they need to apply pressure in a way that they can scale back (seize and return territory is traditional). If they want to conquer, they need to make surrender more attractive than resistance, which is a balancing act.

What end-state they want and what strategy they pick to achieve it should dictate their force structure. For example, if they want to conquer, they should want to make surrender as palatable as possible. So, even if terrifying monstrosities might be better at actually punching dudes to death, your necromancers might want polite and friendly police constructs to do all their fighting. If they want a negotiated settlement, you might have zombie bats bombing the opposing capital.

On an actual battlefield, the same reasoning applies, but on a smaller scale and much more specific to the opposing force. You can see this looking at something like elephants. The first time elephants show up, they might as well be unstoppable. But once you know that the enemy has elephants and have tactics in place to deal with them, elephants are an expensive waste of resources. So, if you're Hannibal, do you bring elephants or no? (Clearly yes, but the trick was bringing them somewhere unexpected).

The way you have constructed this, your undead are just arbitrary smart motivators. Walking ballistae, armored bulldozers, shield walls, suicide firebombs, exoskeleton armor suits, syringe-hornets, etc. are likely to be more productive areas of inquiry than weird flesh-sculptures.

There is also a saying that amateurs study strategy, professionals study logistics. That's true. If you can't get your force to the fight, or if they are ineffective once they get there for lack of beans/bullets/bandages or fuel, the fight is over before it begins. The Roman legions were so effective in large part because they could move fast (ten miles per day!), and had good sustainment. For the opposite example (great fighting with bad logistics), consider how the Napoleonic and Nazi invasions of Russia went.

Of course, none of that matters if your necromancers' egos drive them to do something different.

Finally, combined arms has historically been a very successful approach. So as a default answer, that.