During Missing, Presumed Death, speak with Azzanadra, Wahisietel, or Akthanakos about why they are at the Ascension ceremony, whether or not Sliske is still a Zarosian, whether or not Sliske has ascended, or special dialogue options after completing The World Wakes.
He is the start point of the Fate of the Gods quest, asking for you to help bring Zaros back.