If you explore Vampire Castle, you'll ultimately learn that Lord DeRosso gained his immortality while burning in his own castle's flames – when some dark voice offered him power everlasting.
During the final battle, Lord DeRosso makes an appearance: before sacrificing himself against Lord Ouroboros, he recites the same words as the dark voice – then he fuses with Ouroboros' core.
But, this still doesn't quite explain Lord DeRosso's immortality – nor is it very clear why did he recite those words (obviously he's the dark voice his past self heard, but I still don't see the point).
So, what exactly makes Lord DeRosso immortal?
Best Answer
I actually don't recall directly either; and I'm not sure it was ever clearly explained. It's an interesting chain of events I can't get straight in my head:
So I guess he somehow was able to make himself immortal?
I'm pulling this from the final fantasy wiki: