The Dark Emperor is the dark half of Emperor Mateus's soul and the final Boss in Final Fantasy II.
After getting into Palamecia's mountain-protected castle, the party confronts the Dark Knight. Before they can do anything, the Emperor reappears, in this form. By killing him, the party only made him stronger, as he has now gained incredible strength and became Emperor of Hell. Ricard summons the Wind Drake to get the party out of Palamecia while he confronts the revived Emperor. The Emperor kills the Dragoon easily.
While it is not known if the Emperor intentionally allowed the heroes to kill him so he could gain the dark powers of Hell, given the fact he already had power over Hell-spawn, it is possible that it was his true intention all along. Summoning the castle of Hell, Pandaemonium, where Palamecia once stood, the Emperor intended to reign over both earth and Hell. However, the party traveled through the Jade Passage and confronted the Emperor head on. He is eventually defeated, and returns to Hell.
Dissidia Final Fantasy
The Dark Emperor appears as Emperor Mateus's EX Mode.
Final Fantasy VII
Final Fantasy XIV
Final Fantasy Crystal Chronicles: Echoes of Time
Final Fantasy Chrystal Chronicles: My Life as a King