Castiel, often shortened to Cas, is a powerful angel of the Lord who was responsible for saving Dean Winchester from Hell, on the archangels' command. It was his job to help prevent Lilith from breaking the 66 Seals that, once broken, would release Lucifer from his cage in Hell. Once Castiel learned that the angels wished to free Lucifer anyway, he defied his angelic brethren and died trying to help Dean prevent Sam Winchester from breaking the last seal.