St. Mary's Convent
Located in Ilchester, Maryland, the convent is where the mouth of Lucifer's Cage opens when the 66 Seals are broken. It is nearly impossible for any demon to communicate with Lucifer while he is confined to his cage. Azazel searches, discovers the nature of St. Mary's, and ventures to this convent, possessing the head priest. He manages to speak to his 'father' and receives his instructions. Nearly 40 years later, Lilith comes to the convent. Sam Winchester arrives and kills Lilith, opening the door of Lucifer's cage.
In 1972, Azazel takes control of the body of a priest stationed at St. Mary's Convent. In this guise, Azazel tells eight Nuns in the chapel the story of Lucifer and his fall from the demons' perspective, then ruthlessly slaughters them all with a blade. He places the body of a nun on the altar, and implores Lucifer to speak to him if he can, and tell the demon how to free him. Lucifer speaks through the corpse, telling Azazel that Lilith is needed for his release. He also tells Azazel to find a special child.
Azazel: But, ye of little faith, because I finally found him -- or at least, you know, a spot where his cage door opens. It's right here. In a damn convent, for God's sake. Life is funny.
Nun: Um, Father?Azazel: Shut your friggin' piehole, you little slut! (he grins) Then again, I suppose it makes sense. Folks forget my Daddy is an angel, after all. Or was. I mean, I suppose some dumb bastard stood here, felt a jolt of his holy juice and thought, I'm gonna build me a nun factory. You know? It's the right idea. Wrong angel.
– Azazel to the nuns, 4.22 Lucifer Rising
In 2009, Lilith, having by this time caused 65 of The 66 Seals to be broken, goes to St. Mary's. Knowing Sam Winchester will pursue her, Lilith prepares to meet her fate and makes ready an altar. Sam arrives with Ruby. He telekinetically throws Lilith against the altar and pins her there. Dean arrives to stop Sam, but is prevented by Ruby. Sam, full of [Demon Blood]], kills Lilith. Her death is actually the final seal that will free Lucifer. Lilith’s blood forms a spiral in the middle of the chapel and the gates of Lucifer’s prison open. 4.22 Lucifer Rising
The doors to the chapel close and Sam and Dean are unable to escape. However, before Lucifer appears, Sam and Dean find themselves transported to an airplane flying over the area. Joshua later reveals to Dean and Sam that it is in fact God who saved them from Lucifer's rage. As the plane passes over the site, a column of intense light erupts from the convent.
Later, a radio broadcast discusses speculation that the convent may have been attacked by terrorists.
Legends about St. Mary's College
There is a real St. Mary's College in Ilchester, Maryland, known as the Hell House. Legend has it that a priest hung five young girls facing one another around a pentagram, and then shot himself. The College is believed to be haunted, and at night the girls' screams can be heard.