(Art is not mine!)
Shuichi basically is haunted by demons and different creatures in this AU and his reality is completely distorted. He calls them in a passive way 'Nightmares'. He believes this started happening ever since his first sleep paralysis experience as a kid which scarred him for life.
His main 'Nightmares' are
Trickster!Ouma - tries to scare Saihara, teasing him, saying he will kill him and placing a knife on the neck while also flirting with him. He never actually tries to kill him, only threatens him and makes him believe he's gonna kill him
CultLeader!Angie - Constantly asks him to join their Atua cult every time she appears. If he refuses, she recites some ancient Atua text which suddenly makes Saihara breathe heavily and choke
Psycho!Kaede - Kaede appears often with her piano or with a melodica and plays music as loud as possible, trying to make Shuichi go crazy.
Assassin!Maki - Maki is a nightmare that constantly follows Shuichi and tries to stab him. When she does, Shuichi usually faints or is awoken at night from sleep
Insomnia!Himiko - Himiko isn't a direct nightmare and rarely ever appears in person and if so she appears faintly almost like a ghost. She is simply there to give insomnia to Shuichi in order to help others torture him for longer.
Self-Loathing!Miu - Whenever Shuichi is insecure about himself, Miu shows up just to scream insults at him until he breaks down crying
Anxiety!Kaito -Every time Shuichi tries socializing Kaito comes in and tries overanalyzing all the possible outcomes of what Shuichi says, to freak him out
Heartbroken!Tenko - Tenko rarely ever shows up, but if she does, she keeps screaming at Shuichi that he'll get hurt if he ever tries to get in a relationship
Existentialism!Rantaro - Rantaro often mentions how Shuichi will definitely die alone with nobody to remember him
(That's all the nightmare ideas I have for now but I'll definitely update this later with nightmare versions of other characters!)

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My Danganronpa AUs
FanficAUs that I made for Danganronpa. I used to put AUs in my Headcanon book, but now I have a separate book for that. You can use these AUs for your fanfics or as prompts just remember to credit