In the Plane of Time: For event information, see this quest entry. Tallon Zek drops three items from his loot table.
In the Plane of Tactics: Max hit ~1,000; does not summon; shadowsteps himself around the room. Most of his damage output is from spells. Due to the "Tallon's Balance" debuff he casts, you will need to buff resists to ~450 before being able to resist many of his spell attacks.
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).