Stull Cemetery is a real cemetery in Stull, Kansas about 40 miles from where Sam and Dean were born in Lawrence, Kansas. It has a lot of lore associated with it, including that it may be a gate to Hell.
Lucifer tries to dissuade Michael from the fight, blaming their conflict on God, and suggesting they walk away. Michael counters that he must carry out God's orders, and that it's his destiny to kill Lucifer, who is evil. Just then Dean arrives in the Impala, Def Leppard's Rock of Ages blaring.
Dean tries to talk to Sam, inside Lucifer, but Michael tells him it's not his fight anymore. Castiel arrives, tossing burning Holy Oil on Michael, causing him to vanish temporarily. Lucifer turns and disintegrates Castiel, and then as Bobby shoots him, he snaps his neck.
Lucifer starts to beat Dean, taunting him, saying he will kill him slowly and make Sam feel it. Dean doesn't resist, just telling Sam he won't leave him. As Lucifer goes to land another blow, he glances at the Impala, and suddenly all Sam's memories of life with Dean rush at him, allowing Sam to take control from Lucifer. He tosses the Horsemen's Rings to the ground and recites the incantation to open Lucifer's Cage. He and Dean look at each other one last time, but as Sam goes to throw himself in, Michael reappears. He tries to stop him, but Sam drags him into the cage too, and it shuts.
As Dean grieves, broken and bloody, Castiel reappears and heals him. His angelic powers are restored, and he also resurrects Bobby.