Saving Grace
If you have already saved Tristian in one of the story quests, he’ll seek king’s audience. After having a conversation in a throne room, you’ll need to meet Tristian outside, in the main square. Head for 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 is the First Faithful building the settlement. Go to the City of the Hollow Eyes (in the upper right corner of the map) where you’ve already been when looking for Faceless Sister. Move to the upper corner of the map to find the cultists. Interrupt the ritual, take the enemies out and complete the task.
You may let Tristian leave this area. But if you let him choose what to do, he’ll stay there to free the Stolen Lands. You’ll get 4800 XP after completing the quest.
The location you need is NOT close to the ‘Temple of the Elk’. In fact, it’s in the completely opposite direction. You actually need to go to the ‘Secret Sanctum’ south of ‘Bridge over the Gudrin River’ east of your capital.
Thanks, spent a lot of time searching around the Temple of the Elk