Asking Banvira for duties after the events of Counterintelligence will see her request you to remove a potent wizard and potential threat residing under the volcano, Mount Sarrunabi. Speak to her about Shalath Abkala, and she will have this to say:
"Shalath Abkala is a powerful necromancer known to lurk in the lava caverns beneath Mount Sarrunabi, the volcano northeast of Firewatch. During the Imperial Simulacrum, the necromancer took advantage of the chaos after the Battle of the Inner Sea to gather innumerable corpses for her horde. Shalath has been quiet since all that tumult, but I know for a fact that she's still down there, just as formidable as ever. It's time we do something about it."
Prepare yourself for extensive combat before setting out from Firewatch. Mount Sarrunabi lies close by, to the north of the city. To enter Shalath's den, exit the southern gate of the city and follow the road, taking the first fork to the left. You will find yourself opposite the entrance to the Mount Sarrunabi Cavern - an extensive subterranean landscape. Once inside, you'll quickly reach a fork in the passageway. Either route will take you to the central chamber. An Invisibility potion or spell will allow you to bypass the bulk of the fighting, as Shalath has amassed quite the army of former legionaries.
Kill Shalath in the central chamber - she is not entirely immune to silence spells, and has no weapons otherwise. Then return to Banvira and ask about Shalath Abkala for your reward.
