Astarion says: "I'd rather be the only dark power inside your body"
Durge scene from act 2 if you refuses to kill Isobel and are in a romance with Astarion. Them Astarion steps in to help his beloved Tav resist her dark urges. Astarion's support becomes crucial in this moment.
This video includes almost every outcome from the dialogues with Astarions.