Elidibs was a hero from the Fifty Years' War, the greatest wizard to come out of Gariland Magic City. What happenes to him in between then and now is not stated in the game. Somehow he found an extra Zodiac stone (most likely in Deep Dungeon itself).

Ramza and friends interrupt his mediation in the END level. I seem to recall he senses the other Zodiac stones they carry and turns into Serepentarius to take them by force. Nothing else is known about Elidibs. Even his description in the Brave Story seems at a loss to elaborate further.