In the Plane of Time: For information about this event, see this quest entry. Vallon Zek drops three items from his loot table.
In the Plane of Tactics:
In the Rathe Council Chambers: A chest with loot spawns after the Zek Brothers event.
In the Plane of War: Part of the 15th anniversary raid event "The Brothers Zek" (see this quest entry for information).