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.