Lucifer's Cage

Lucifer's Cage was a special part of Hell in which the Archangel Lucifer, or the Devil, was incarcerated following his disobedience to God. Its entrance is initially located in Ilchester, Maryland, beneath the site upon which St. Mary's Convent was built. The door to this part of Hell was protected by the 66 Seals, which acted as locks that kept Lucifer from escaping unchecked. The Yellow-Eyed Demon, Azazel, eventually discovered the prison's location and set into motion a plan to break the Seals and free Lucifer to walk the Earth and bring about the Apocalypse. Several decades later, this plan comes to fruition in Lucifer Rising when Sam Winchester follows Lilith to the convent and kills her there, breaking the final Seal and opening the door to Lucifer's cage in Hell, allowing him to escape into the world once again. It is revealed in Hammer of the Gods by the deceased Gabriel in a farewell message to the Winchesters that the cage still exists down in the depths of Hell and that Lucifer can be re-imprisoned there, but the only way to do this would be to acquire the cage's four keys: the rings of the Four Horsemen of the Apocalypse. The rings act as the cage's keys and with them, the Devil can be returned to his prison in Hell if he can be lured back into it.

In Swan Song, Michael (possessing Adam Milligan) and Lucifer (possessing Sam Winchester) both fall into the cage after Sam regains control of his body long enough to activate the rings of the Four Horsemen of the Apocalypse and reopen the portal to the cage. Though he fell into the cage with Lucifer in tow, Sam is returned to the Earth at the end of the episode, presumably through God's work as only he seems to possess the power to do something of that nature without reopening the cage.