Bosses
Andariel
Greater boss in Kehjistan. Hoard opens with a Greater Lair Key.
Where and how
Her lair is Hanged Man's Hall, east of Tarsarak in Kehjistan. The hoard takes one Greater Lair Key.
Unique pool
Mythic drops from this hoard come from this pool. See Lair bosses for how mythics work.
Beastfall Boots and Deathmask of Nirmitruq (both Rogue) can also drop here, and Beastfall Boots can drop anywhere else too.