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Yes, you read that correctly. My villain character is literally a virus. He thinks and behaves just like any other character does, mentally and
personality-wise, but his body is best described as a wispy, dark cloud. He enters people's bodies and takes over their minds and actions. No, I haven't lost my mind. It's a sci-fi setting. The virus is actually an alien life-form.

The problem is how do I make a wispy cloud menacing? How do I create actions against a wispy cloud? If he takes over other people's minds then wouldn't those people be the ones who are menacing instead of him?

In the story, this virus takes over the minds of prominent people in power and makes them do its evil bidding. I created it this way so that it would be undetectable to my hero characters, and so that it could take over many different life-forms so that it can seem as if the threat to my hero characters can come from anywhere.

But how to write it in such a way that it comes across as menacing?

First, it appears that your character is a parasite, not a virus. Viruses are mostly about replicating and spreading to more and more hosts. Your character seems to simply move from one host to another.

As far as the writing, it is all about actions and motivations. What does the main character want to do and how far is it willing to go to accomplish that? Who wants to stop him and how far are they willing to go to stop him?

Are the character's goals or methods menacing? Are the other characters sympathetic (to build pathos when the main character takes action against them)? Do the other characters know that the main character exists? How did/do they find out? What can they do to stop it?

It doesn't seem to me that the main character is too different from any other character except in the trappings.

Just as a note, what you are talking about is not a virus. It is more akin to a parasite, although not necessarily, as a form of resource-stealing from the host is required for a parasite status. Also, great question!

The fact that anyone, everything from your best friend, to the president, to your pet, can be under control of the villain is quite terrifying. But, in a narrative, it might not be so terrifying to the reader. The reason for this is because for a character to be menacing, it needs a "face".

When I say face, I don't actually mean a face. It can be that, of course, and it works well. But in your case, that wont be too easy, as the villain will probably most of the time be inhabiting people, and when it is isn't, it just a wispy cloud.

So, what will your villain's face be then? It can be anything, as long as it is of him. Let me give an example; The movie Bird Box had some of the most terrifying villains in my opinion. And they didn't even have an appearance (at least one that was shown to the audience). So, what was their face of menace? Their effect on the environment.

Whenever someone looked at them, their eyes turned black, and they promptly killed themselves. The first scene when the main character is sitting in the car with her sister, and set sister looks at the "monster", and then kills herself, is perhaps the scariest scene I have seen in my life. Why is this?

Because the sister kills herself. She does perhaps the most drastic thing a human can do to their own body, and she does it because of the villain. She doesn't do it out of her will, the villain makes it her will. And that is scary, because one of the things we humans value highest is our own freedom to do what we chose. To do what we want to. And when what we want to do is suddenly in someone else's hands, that is scary.

So, the face of the Bird Box villain is their ability to make anyone kill themselves just from a glance upon them. Something which helps to this effect is the aesthetic part. They eyes turns black, which is always a chilling sight. Also, their presence is to be experienced. Though they cannot directly hurt any of the characters, their physical impact on the world gives the illusion that they can, the physical impact being wind picking up leaves and moving trees and bushes.

Obviously, since undetectable is key for your villain, you can't really have an aesthetic element to the mind control itself. But if you're able to, try to sneak in something aesthetic. Something left behind, perhaps were the victim was possessed.

As for the face of your villain, from the outside, it looks to be the fact that it can control the minds of whom they possess. That is scary, for the same reasons that the villains are scary in Bird Box. Lack of personal control. So, milk this. Stress it. Implant this scary thought into the reader's mind, and let the fear it generates grow with the progression of the narrative. You can implant this idea with the 3rd-person or 1st-person narration, or with side-characters talking about it in great detail.

Giving your villain a face isn't the only thing needed to make them menacing though. You need to let your reader know of their motivations, and the possible consequences if they win. Possible is key, because knowing exactly what will happen isn't always good. It can work, as with for example Avengers: Infinity War, where we all know what will happen if Thanos snaps his fingers. But keeping some ambiguity to exactly what will happen if the villain wins can be very effective, and it's all because of one thing: the fear of the unknown.

And this is perhaps the biggest strength you will have when you're establishing your villain. People have strong imaginations, and because of this, any danger that is unknown turns on the anxiety parts of the brain that start creating all these different kinds of scary possibilities. Because you can't see your villain. Your villain exists inside people, and so the reader can only "see" the villain's actions, and the consequences thereafter.

Other than that, it is the interactions between characters and the villain that builds up their scariness. As they are inside people and probably want to stay undetected, this becomes a little tricky. But you can make it work. Have the villain visit characters in their dreams, or talk to them via telepathy. Or maybe have a character talk to a possessed person, before either that person is disposed of or the character is killed, as a result of the cover being blown.

The bottom line is, if you're creative and actively think about what is scary, then you'll intuitively make your villain scary using good and refreshing methods. That is what is most important.

You don't show a "wispy cloud" you just show ordinary people behaving suddenly out-of-character and attempting to kill (or whatever). What could be more menacing than believing anyone could turn against you at anytime for no reason?

Leave the "wispy cloud" for the last scene as it escapes into space.

you really aren't describing a virus..... seems more like a parasite, or hive mind bug.. or some other kind of biological "weapon"

Invasion of the body snatcher..

https://en.wikipedia.org/wiki/Invasion_of_the_Body_Snatchers_(1978_film)

Star Trek had a great job with the idea...

https://en.wikipedia.org/wiki/Conspiracy_(Star_Trek:_The_Next_Generation)

Now if you are describing a CHARACTER as in a single consciousness .. .then you are describing a head necromancer type (like the night king from the Game of Thrones)

I am not phoo phooing your idea.. I am simply pointing out a lot of these villains have been done very very well...

I think that with virus you mean its some sort of inorganic life form programmed to destroy.
A way to make it menacing is showing the victim's point of view. How do they feel when wispy cloud enters the body?

Do they black out or feel sick?

Do they remain aware as a paralyzed witness while the virus hijacks their bodies?

I had a terrifying nightmare where a purple ghostly ball attacked me.

What made it terrifying?

The unknown and weird and how its contact was bone-chilling.

Think about how in The Ring the glitching videotape is terrifying without demons, gore and the like.

You can check out the Code Lyoko series. There, a malevolent virus named XANA tries to kill and/or possess the main characters every once in a while.

What's his story development? His attacks gradually become more and more sophisticated, starting with overtaking machines to actually creating an army of robots. He becomes stronger and stronger, and is at times almost invincible.