Key to the Bunker
The key belonged to the Men of Letters and was kept in a box which was decorated with their crest - the Aquarian Star. Inside the box is "the key to every object, scroll, and spell ever collected, for thousands of years, under one roof". Basically, the Men of Letters had stored, in one location, all the supernatural knowledge that they had ever possessed. If the Box were to be hunted down by any "evil being" (demons, monsters, etc...) it would be safe when stored in the area where they had placed all their scrolls and secrets because the area itself is branded with symbols to ward off every kind of evil ever created.
Abaddon killed David Ackers and Ted Bowen, and blinded Larry Ganem. After Henry bursts through to find out what was happening, Larry crawls up to Henry and entrusts him with a box which contains the key to where all the knowledge the Men of Letters has collected is stored. Henry goes through a portal created by his blood spell, and is followed by Abbadon.
Henry meets up with Sam and Dean, and together they manage to track down the sole survivor of the massacre.
Sam visits Larry Ganem to find out more about the box and its contents. After Larry explains everything, Abaddon, who has possessed Larry's wife and was listening to the entire conversation, kills Larry and captures Sam. Abaddon then calls Dean to demand a trade: Sam for Henry & the key. During the trade, Henry shoots Abaddon in the head with a bullet that had a devil's trap carved into it. This immobilizes Abaddon where she stands, and she screams "Why am I stuck?!" in frustration. Dean then decapitates her and says that he will cut her up into strips and bury her. After Henry's death, Sam and Dean decide to find out if the Bunker is still intact.
Sam and Dean follow the coordinates provided by Larry, and find an underground Bunker, which they are able to enter using the key.
Dean hands Kevin the key to the Men of Letters Bunker where he can be safely protected.
Mick Davies told Sam and Dean that the key to the bunker can open every other bunker scattered across the world.
After arriving at the Men of Letters' Rhode Island Chapter, Sam and Dean find a man-hole cover with the Aquarian Star in the center and a key hole. Sam uses the key to open the hatch and gain access to the hallways beneath the building.