Bosses
Varshan
Initiate boss in Hawezar. Hoard opens with a Lair Key.
Where and how
His lair is the Malignant Burrow, next to the Tree of Whispers waypoint in Hawezar. The hoard takes one Lair Key, and killing him also gives a Greater Lair Key.
Unique pool
Mythic drops from this hoard come from this pool. See Lair bosses for how mythics work.
Vox Omnium (Sorcerer) can drop here or from the Beast in the Ice.