This is the very last zone in Reaper of Souls, and entering it starts the final battle with Malthael. Before it, there is a small bridge overlooking the Heart, the last zone in which the Nephalem can use the Town Portal. The battle takes place on a small platform that hangs on chains, literally inside the Crucible. In the ending cinematic, one may see that the Heart is formed around the structure with roof (which is destroyed by the blast).