Bosses
Astaroth
End boss of Escalating Nightmares, with his own unique pool.
Where and how
Astaroth waits at the end of an Escalating Nightmare, which you open with an Escalation Sigil. His loot uses his own unique pool.
Unique pool
Mythics from Astaroth follow the same rules as lair bosses.
Rod of Kepeleke (Spiritborn) can drop here as well as from Duriel.