"Pandemonium Lair Boss Key" for the July 14th patch.

"Combining a Lair Boss Key with a Pandemonium Fragment in the Horadric Cube guarantees a Mythic Unique from the Unique Drop table of the Lair Boss chest you open."

You've got 4 days until the patch on the 14th to implement this Blizzard, that's plenty of time, get it in the game.