The monastery was once a proud bastion of Lordaeron's priesthood - a center for learning and enlightenment. With the rise of the undead Scourge during the Third War, the peaceful monastery was converted into a stronghold of the fanatical Scarlet Crusade. The crusaders are intolerant of all non-human races, regardless of alliance or affiliation. They believe that any and all outsiders are potential carriers of the undead plague - and must be destroyed.
Reports indicate that adventurers who enter the monastery are forced to contend with Scarlet Commander Mograine - who commands a large garrison of fanatically devoted warriors. However, the monastery's true master is High Inquisitor Whitemane - a fearsome priestess whose guardians seem difficult, if not impossible, to fell in battle.
Despite Scarlet Monastery Graveyard having been removed in patch 5.0.4 and merged into a new dungeon, the old map, terrain, and objects were used in 2012's Hallow's End. The holiday's boss, Headless Horseman, exists in a special version of the dungeon in which its normal denizens are not present. Thus, it may be presumed that the use of the old dungeon simply was an oversight by Blizzard, as 2013's Hallow's End utilized the correct (i.e.: updated for Mists of Pandaria) dungeon.