Bosses
Grigoire
Initiate boss in the Dry Steppes. Hoard opens with a Lair Key.
Where and how
His lair is the Hall of the Penitent, south of Ked Bardu in the Dry Steppes. 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.
The Oculus (Sorcerer) and Sunstained War-Crozier (Spiritborn) can also drop here, as well as anywhere else.