“Make your way to the lower levels of the Hellmouth.”
Immediately when you touch down, you are in the Summoning Pits. Jump up to the arena above you and Malok will appear. He will give out a roar and summon in a number of Taken that you will need to fight through. Malok cannot be hurt right now so ignore him and just focus on the Taken that come on in after you.
Having cleared out the Taken, go to the back right and head for the door there leading into the passages beyond. Of course, the door is sealed with Hive Runes. Time to search for the TombHusk that are scattered around the area.