Once the Narsis section is merged to mainland, edit Q6 of the Almas Thirr MG questline to point the player to Bessarnamidan instead, which is where the quest dridrea Yathosil will be. A copy of Yathosil is already placed there in the CB NPCing claim. Remove the existing Yathosil from Balititashpi, and if needed replace them with a non-dridrea daedra (if the room needs an enemy).

We ~might~ have new spider daedra by this point, but that's a different issue.