Gaster Mickey is fan made easter egg antagonist in A:DI.
Gaster Mickey is almost like True Mickey, but with look of W. D. Gaster from Undertale. When typing GASTER on the keyboard on night 1, he saying "Who are you calling for?" and HE begins the battle instead of True Mickey, when trying to get the true ending. On the sixth night the power turns off automatically, the flashlight breaks. Then his dialogue begin.
"Well, here I am. Were you calling me? If so, give me something I want, and I will give you something you want. All I want now is YOUR SOUL. Give it to me, and we have a deal."
After his dialog ends, you can choose if you should give him your soul, or not. When selecting yes, Gaster Mickey will jumpscare you and erease all your game's data. But if you said no...
"I see now. I guess you don't actually need me. Wait. No. I actually really need it. I want to be finally free. I can guess that I have to take it by force."
Then, the battle begins. Truer Than Fiction was playing during the battle. Once dying, Gaster Mickey jumpscares you. When you finally beat him...
"WHAT? HOW? THIS CAN'T BE. THIS IS WHAT HAVE YOU DONE. IT'S ALL YOUR FAULT. YOU SHOULDN'T HAVE DONE WHAT YOU DID. I SHOULD BE--"
Then the game crashes. BECAUSE YOU'VE DONE THIS. Once coming back, you recieve his content in the extra menu. But instead of the title on the main menu, the title changes to "WHY".