Abbey of the Road

The  is a Christian monastery somewhere west of the Commonwealth. It sends out missionaries to spread the faith and Marcella, one of these, can be met in Point Lookout. It also retrieves religious relics, but that is a secondary objective. It's location is secret, and if you ask Marcella the location, she will refuse.

Appearances
The does not appear in any game but it is mentioned in the Fallout 3 add-on Point Lookout by Marcella who claims to be a missionary from there.

Behind the scenes
This could be a reference to Abbey Road Studios, the recording studio of the Beatles and many other artists.