Nightmare is the True Main Antagonist of FNAF 4 and It's Series.He is the Child that is the Victim of 83.
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Trivia
He is Similair to Few Villains
Venom: Both Wear Black and Seek Revenge
Scar: Both are Younger Brother of the Hero, Both are Tragic,Both hate Their Brother,Both are Fallen Heroes,Both are Complete Monsters.
Chernabog:Both are Devils
Hades:Both are Immortals
Egon:Both are Fathers of the Protagonist.