Replying...
Intro. Remember.... this is from INDIE CROSS so if you remember so yeah a strange ritual was around Cuphead and it was infecting him or making him evil and Cuphead opens a red portal leading to the devil so yeah this what I could explain it's from nightmare Cuphead vs the devil video so I'ma say it from the beginning when the devil and the nightmare Cuphead started fighting so here you go

Nightmare Cuphead vs the devil

@Ultrakill