If I recall correctly, there are two parts of this "quest":
- Going to the island where you and your brother lived as kids
- Meeting your brother for a confrontation
Both of them are marked as quests and give a specific map indicator. Until then, I think you just have to progress other quests.
I am trying to complete the "The Gatekeeper" and "Secretly" quests. I already confronted Kallax once, and am I guess building up toward a second confrontation. (I've been to the island already too.) But I am unable to proceed further and am stuck. The "The Gatekeeper" has two little counters below it with Wardek and Thorald's names listed, but talking to both of them only increments Wardek's counter.
Well, the second confrontation is where Zardom lives. Can always check there, but I'm pretty sure it triggers after completing just about everything else, including taking down all the converters, trying the ritual with Thorald, rescuing the girl etc.