The Heart of the Damned is a demonic artifact and the source of power for Azmodan's Sin Towers. The Heart of the Damned is one of the two known Sin Hearts.
When the Lord of Sin launched his assault on Bastion's Keep, he dredged the Sin Hearts from the depths of his realm in Hell and brought them forth into Sanctuary to empower his demonic legions.
Azmodan's consort, Cydaea, guards the Heart while his commanders are fighting in the field. It is very vulnerable, and without a shield to protect it, the heart can be crushed easily. The Nephalem must destroy it to reach The Core of Arreat and kill Azmodan.
In game
Destroying the Heart of the Damned is required for the quest Heart of Sin. The Heart of the Damned (object) is located in the Heart of the Damned (area), deep within the Tower of the Damned.
Upon entering the central room in the Heart of the Damned, Cydaea confronts the player(s). The player(s) needs to weaken Cydaea in order to dispel the shield surrounding the Heart, which allows the player(s) to destroy it.