I left the ancient Argonian ruins with a single Fang of Sithis. According to Looks-Under-Rocks, the Renrijra pirates are in an uproar. They may be feeling the effects of the other Fang in their midst.
Board the Pirates Ship
I must search the pirate ship. Looks-Under-Rocks heard screams. I should look for the ship's captain.
Recover the Fang of Sithis
There are fires belowdecks, and I can see why. Captain Nahrevii has been using the second Fang of Sithis on her own crew. Time to take it away from her.
Like Barvyn, the pirate captain was driven mad by the Fang of Sithis. I defeated her and recovered the second Fang. Now I should find Looks-Under-Rocks at his camp.
— game journal
This quest is closely related to The Fangs of Sithis. When one returns to the camp, an Argonian woman named Sharava is waiting, claiming the Fangs are a property of the Night Mother and belong to the The Dark Brotherhood.