This quest is obtained by examining a book, Discourse on Extra Planar Entities, that is found in the entrance hall to the Iron Fortress.
Collect 9 Corrupted Skulls from various undead in the Iron Fortress. After collecting the skulls place them on the fire on the bottom level of the Fortresses first northern tower. This will summon Legul so you can put an early end to him before Jakub can utilize his evil powers. Head back to Denegar's Stand and speak with Philobian Mikkarz to receive a reward for your efforts.