The Xenos became a threat to the Imperium, when it used its immense psychic abilities and the power of the Warp, to corrupt the population of three Imperial Systems into the worship of the Chaos Gods. However its reign of terror finally ended, when Marest and the Blood Angels killed the Octocalvariae's followers, but they discovered the Xenos itself was unkillable. Its corrupt nature healed any wound dealt to the Octocalvariae and only at great cost to the Blood Angels were they able to capture and bind the Xenos. Marest himself was mortally wounded in battle with the Xenos, but before he was put within a life saving Sarcophagus, he ordered that a prison be constructed that could hold the Octocalvariae. His orders were immediately followed and the prison was built into the Carceri Arcanum tunnels, which lie beneath the Blood Angels' Fortress Monastery and was named the Vault of Marest. The Vault now contains Chaos creatures and artefacts that have come into the Blood Angels' possession and are either too dangerous to destroy or, as in the case of the Octocalvariae, are undestructible. The prison has only one entrance and exit which is guarded by Marest, who is now interred within a Librarian Dreadnought and has maintained a vigil over the prison for three thousand years. Marest will only allow high ranking members of the Chapter to enter the prison, but though he receives few visitors, he has not been forgotten by the Blood Angels. The Chapter teaches Marest's history to each generation of Neophytes and in doing so, they ensure that every Battle Brother knows of his sacrifice and eternal dedication to the Blood Angels.