This quest is acquired from Sigrid, who can be found in a hut south of Ridgeline Camp. She has exiled herself from Stone-Bear Hold, but she won’t tell you why. After you’ve spoken with her, go to the hold to learn more about the situation. In particular, you should speak directly with the Augur to learn the most relevant information. After speaking with him, take the Lakeside Road to the south and visit the ritual site. You will find various pieces of evidence at the shore by using the search function when you’re near to them. Once you’ve gathered all the evidence, return to Sigrid and confront her. You will then have the option to recruit her into the inquisition, or encourage her to confess to the Augur.
Note: If you choose to recrit Sigrid, Cassandra, Varric, Sera, Iron Bull, and Vivviene will Slightly Disapprove, while Cole will Slightly Approve.