If you have saved Tristian in one of the story quests, then he’ll seek out king’s audience. After having a conversation at a living space, you’ll need to fulfill Tristian outside, at the main square. Head to the Kingdom of the Cleansed — it’s not far from the Temple of the Elk.

Kill the cultists and investigate the bodies to get to know where’s the First Faithful building the payoff.

Go to the City of the Hollow Eyes (in the upper right corner of the map) where you’ve already been when searching for Faceless Sister.

Move to the upper corner of this map to find the cultists. Interrupt the ritual, take out the enemies and finish the undertaking.

You might let Tristian leave this place. But if you let him choose what to do, he will stay there to spare the Stolen Lands. You will get 4800 XP after finishing the quest.