The fight will occur at coordinates 71,35. There are 3 elites who are channeling the Heart. If you attack them, they will not move but a wave of non-elites will enter the fight. Once all 3 channelers are dead, Deathwail will drop down and take the Heart. He can fear, deathcoil, and shadowbolt if you stay at range. The Heart will drop off his corpse.
Some people report that it's possible to land on top of the building with a ranged character and attack the channelers. The wave of non-elites will not "glitch" themselves on top of the building to attack you. If you wait long enough, they'll eventually despawn. However, this could be considered an exploit by Blizzard and adventurers should do this at their own risk.
Edit: This has been fixed by Blizzard, if you attempt to stay on the building, the non-elites will attack from range with bows.
It has been known to us that the fel orcs, under Illidan's command, have guarded an Ata'mal crystal known as the Heart of Fury at the Ata'mal Terrace. They cloud its presence with dark magic, to keep it from the Sha'tar.
Fight your way to the Atam'al Terrace outside the Black Temple and turn your eyes to the sky. Shadowlord Deathwail will circle the terrace, beyond your reach. You will have to slay him and his warlocks before you can obtain the crystal.