Act 3, Secheron waypoint, go into Elder Sanctum, find Immortal Throne, voila!
Some say you need the Staff of Herding, some say you don’t (I had it on me, just in case). EDIT: You don’t need Staff of Herding, checked with my seasonal toon now!
Also, disregard difficulty, I just picked a non-season char without amulet and leg gems with wrong skills and all, just wanted to check it.
Kudos goes to leon0king:
http://www.diablofans.com/forums/diab…
Sadly, the level is kinda lackluster, no chests, not even bosses.