Not sure if this is a bug, or it is meant to work this way… But weirdly, despite not even meeting/killing any undead in my session, when I went in to finish the Malkaddar questline, and started to defend the necromancer doing the ritual - suddenly Moosilauke showed up in the room with the Order of Death’s Vigil merchants. Luckily, either he did not get triggered, or the locked grate prevented him from joining all the undead being summoned in the ritual. So afterwards, I walked out and got attacked by him and his armies of Frost Revenants, and the Death’s Vigil guards just stood there and did nothing. Ironically, his second form died right over the ritual chamber’s font!
Anyway, just posting this in case he is not really supposed to spawn in here.