The Demon Prince

Note this takes place AFTER The "Just My Boss".
After Nightmare fell into the crack he finds himself at the place where he was born. His people waiting for his arrival to see is he will be crowned as their King.
Nightmare Fredbear in the over world has spent days and days on trying to get to Nightmare until he found someone who can help him. With the person's help he was able to go to the under world where Nightmare was at. But Fredbear didn't see the same Nightmare he knew years ago. He saw someone else and tried to get his Nightmare back.