Brotherhood of the Watch
The Brotherhood of the Watch, also known as the Order of the Watch, is an Imperial cult that was formed by the Ordo Malleus millennia ago.
During the Great Crusade, when the armies of the Emperor uncovered worlds that worshipped foul Daemons, those planets that worshipped abominable gods were cleansed or cast into the Warp. However, some Daemons could not be destroyed or banished, and instead were imprisoned on their worlds, held in place by psychic wards. Later, the Ordo Malleus created a secret order of guardians to watch over these prison worlds. By the 41st Millennium, the Administratum was hardly aware of the Brotherhood of the Watch, and the nature of their mission meant that they were unable to inform others of their sacred duty.
Eventually, a squad of the Blood Angels Chapter under the command of Captain Leonatos arrived on the world, unaware of the daemonic presence, until one of their Scouts was abducted and sacrificed. This forced Leonatos and his Marines to purge the region, but they were unable to stop the escape of the Daemon Prince. The Blood Angels departed, the world, leaving behind a sentinel beacon, and declared the world Mundis Exocommunicata until the Ordo Malleus returned to cleanse it.