Echo of Varshan (Initiate Lair Boss, 1x Lair Key) - dedicated multi-class Unique
General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table
Can also be crafted into a Unique Charm (Horadric Cube > Craft Unique Charm) per 3.2.1 notes; any Unique Charm has a small chance to drop at Mythic quality.