The Heart of Sin is an area deep within Arreat Crater. It lies at the bottom of the Core of Arreat and is the seat of power of the Lord of Sin, Azmodan.
The Heart of Sin is the lowest level of the Arreat Crater in Act III of Diablo III. It serves as the 'boss room' of the Act Boss, Azmodan. It is here the player faces the Lord of Sin.