Start NPC: Pelsus
We're here to keep the undead in their place. To me, that means dead, but in the broader sense it means they shouldn't be coming up out of their cave.
If you can get rid of the ossugon prefects, then I think you'll be able to quell the rest of them for a while.
Oh! And don't forget their skulls. They can't reanimate without them.
Excellent. They no longer pose a threat as long as they're in pieces like this.
Hopefully the rest will stay put.
Do you want to see them up and running around outside of that cave?
Kill ossugon prefects and collect their skulls.
Bring the skulls to Friele at the Utterdark Outpost in the Halls of Utterdark.