My favorite (and most hated) monster in the Game. Here are some headcanons I have for him:

-Fiend was experimented on before the events of Monstrum. I'm thinking these experiments were to test the limits of the human brain and see if you could increase its power. Kinda like Eleven from Stranger Things
-These experiments changed fiend's physical appearance, making his body more gruesome and weak in the process, but strengthened his mind to the point he was able to use telekinesis
-Fiend is the way he is in the game because of the bad conditions he was put in during those experiments and hurting/killing other people is probably the only way he knows how to cope/deal with the anger he has built up inside
-Fiend sounds like he's struggling to breath and wheezes (if you really listen to how he sounds, you'd understand what I'm talking about) because the experiments rendered his body weak and brittle, even his organs except for his brain.
-His sharp claws are a last defense incase his telekinesis doesn't kill you
-Fiend can throw things at you or use items to kill you, he just doesn't because he'd rather use his telekinesis and finds it much easier
-Fiend is smarter than the player, he just acts like he's dumb or can be tricked in order to lure the player into a false sense of security
-Fiend chooses to make electronics flicker in order to send the player into a panic
-Fiend doesn't move fast in order to appear weak, or it's just he's literally carrying his body with his mind in order to move
-He can hijack cameras, hence why they don't go off when he's around besides Fiend being able to shut off electronics anyway
-Fiend can talk and I'm 90% sure he would be saying fuck all the time