All the gods got kicked down to mortal status during the Time of Troubles. When a few of them died, a few mortals were able to up themselves to god status. Mystra and Cyric quickly rose to very very powerful status, mostly because they took the portfolios of really used gods (magic, for instance). I believe he also took the domain of Deceit. In the end it's huge Mary Suedom, but explained partially with the concept that, as a mortal he brought some freshness to the pantheon, so he had a lot of ambition and new ideas that the old gods, set in their ways, did not and weren't as able to prepare against. Plus when he -was- a god, Bane, Bhaal and Myrkul were all dead, allowing him to reign around near unopposed as head of evil gods.