Spawn location for "poshadow.txt" map file (note for all Plane of Shadow named map locs, see this quest entry):
Spawns adds at 80%, 70%, 50%, and 20% (two per wave). These adds each have a melee slow aura (Sticky Tendrils) and cast a spell on random targets that flings the target to their location.
Kaas Thox is also an optional kill in the group mission "Breaking the Seal" (see this quest entry for details).