Diablo Wiki
HoF inner

Heart of the Fortress inner platform

The Heart of the Fortress is a zone in Act V of Diablo III. It is, in fact, the largest Soul Crucible in all of Creation, powered by the souls of the dead and built around the inner citadel. Malthael is found within, waiting for the Nephalem for the final confrontation. Killing him disperses the Heart (one can notice the background changing).

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).