A certain number of mobs must be killed before he appears. Normally if you don't kill enough, there will be NO MOB at all in that location, then when you kill enough, either Bael spawns or a regular mob spawns.
You do not need to clear all mobs. I know for sure you just have to clear the mobs after the bridge in Windmoore Harbor excluding Sully for Bael or the regular mob to appear.
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