It's almost time to leave the isle for me, and Sebille's quest is still not advanced. This happened because I had to kill Griff to advance the Red Prince's questline. Those should really have been handled better...
Anyhow, Griff is dead, a body part from him didn't advance the quest, and neither did Zaleskar's Withered Heart. Sebille didn't even have any special dialogue with Zaleskar, ever, just the regular stuff about the ancients rising.
Kinda sucks that you create a situation at the beginning of the game where important quests cancel each other out/endanger each other...