The god that the Atal'ai trolls had brought forth is to blame for this.  We destroyed the god's physical manifestation and sunk the temple, but the god was much stronger than even we had anticipated.  Its seed of corruption ran deep, twisting everything in its path.$B$BEranikus was charged to watch over the sunken temple to ensure that neither the simple Atal'ai nor their god would rise again.  He was an unwitting victim of this corruption, and we in underestimating its potency.