In the Plane of Fear: The God of Fear, Cazic Thule, wanders the northeastern area of his home Plane around the ring of fire. He (unlike his denizens) regards Iksars as his allies. (Screenshot by Kyter)
In the Plane of Time: For event information, see this quest entry. Cazic drops three items from his loot table.
In House of Thule, Upper Floors: Cazic Thule is in a deep sleep, unattackable.
In Fear Itself (group mission): Spawns as part of the task "An Offering to Thule".
In Fear Itself (raid): Cazic is the main encounter in the "Cazic's Final Stand" raid event (see this quest entry for event and loot information).