Lucifer

Lucifer is the name of the supposed penultimate evil to be faced by Dean Winchester, his brother Sam Winchester, and all hunters at large. He is the ruler of the domain of Hell as well as the chief enemy of God and his angels. The entire fourth season of the series chronicles the efforts of Lucifer's associates to break the 66 Seals and release Lucifer himself from his long imprisonment to wreak havoc on the world and once more challenge God. By the end of the fourth season, the final seal, the death of the first demon that Lucifer created after his falling out with God, Lilith, is broken by Sam Winchester (unaware at the time of what this would in fact break the final seal rather than prevent it), allowing Lucifer to finally return to his full strength once again.