Supernatural (Season 4)

 Dean is rescued from Hell and brought back by an angel of the Lord named Castiel, one of several angels who appears throughout the season. The rest of the season follows the brothers as they work with Castiel to stop Lilith's plan of breaking the 66 seals, which would allow the fallen archangel Lucifer, AKA "the Devil" or "Satan himself", to walk the Earth free once again. The relationship between Sam and Dean is strained. Sam starts siding with Ruby over Dean. He begins to give into his demonic side by drinking demon blood to become strong enough to defeat Lilith. He and Dean eventually have a falling out. Sam sides fully with Ruby in his obsessive quest to kill Lilith. Dean makes a deal with the angels to save Sam, and learns that the angels want the Apocalypse to occur in order to rebuild Paradise on Earth. With aid from Castiel, Dean escapes and tries to stop Sam after learning that Lilith is, in fact, the last seal, but Sam kills her anyway, breaking open Lucifer's prison. Ruby reveals her true colors as a demon loyal to Lucifer and Dean kills her. As the season ends, Lucifer's cage opens and he escapes from Hell.