It's really not as weird as it sounds in context. Remember Yu-Gi-Oh! already established that all the monsters really exist and there were always demons, vampires etc as monsters so an actual vampire turning up really isn't that strange. As for "space Hitler" that's the Supreme King/Haou whom Judai is the reincarnation of (just like Yugi was the reincarnation of Atem), that's why Judai has the powers of darkness in the first place. Haou was awakened when Judai's friends were seemingly killed in front of him and, to prevent something like that from happening again, he commited genocides against the duel spirit inhabitants of that dimension, in order to use their energy to create Super Polymerization. Obviously Haou went way overboard and it's a constant source of guilt for our boy going forward, which is largely the reason the last duel is about Yugi/Atem trying to help Judai restore his love of the game and dueling for dueling's sake and why it doesn't matter who won. I'm also...