The Worldstone Keep is the last building, protected by three Ancients, where Baal is. The entrance is sealed until those Ancients are defeated. The most fearsome Oblivion Knights can be found there at Nightmare Difficulty and Hell Difficulty. It is also one of the area level 85 zones on Hell added in patch 1.10, meaning the monsters there have a chance of dropping high level items compared to other places - however, due to the Willowisps and Oblivion Knights it might not be the easiest place to do Magic Find runs.