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.
Destroying the Heart of the Cursed is required for the questHeart of Sin. The Heart of the Cursed (object) is located in the Heart of the Cursed (area), deep within the Tower of the Cursed.
Upon entering the central room in the Heart of the Cursed, Cydaea confronts the player(s). The player(s) needs to defeat Cydaea in order to dispel the shield surrounding the Heart, which allows the player(s) to destroy it.