In the Crystal Caverns: Drops loot and plat. Highly resistant to cold and fire; perhaps immune? to disease and poison. Magic spells land normally.
In the Wakening Lands: Drops little loot and no plat.
In Crystal Caverns: Fragment of Fear: They are in the rock tunnel with Kodik (Quest) who is on Find (Ctrl+F) at -420, 172, -511.
In Crystal Caverns: Fragment of Fear (raid instance): Spawns as part of the raid event "Danela's Stand" (see this quest entry for information).
In Crystal Caverns (ToV):