The Onstantine Priests are mutated Mages, created and manipulated by Supreme Tri-Mage Wilhelm Ryan to do his bidding. There are a total of seven Priests, created from the Mages who survived the Mage War outside of Wilhelm Ryan himself. The Priests initially look similar in appearance to the Mages, before mutating further into unique creatures with unique appearances and abilities.
Wilhelm Ryan first releases the Priests in an effort to complete the mission failed by General Mayo Deftinwolf. Mayo's mission to kill the Kilgannon children suffered two mistakes: Claudio Kilgannon, the oldest son of Coheed and Cambria, was left alive, and Patrick McCormick was a witness to the murder of Josephine Kilgannon and so both of them needed to be eliminated.
When one Priest attempts to kill Claudio, he retaliates by striking a blow into it's head with a hammer, but it stands up, unaffected by the attack. Claudio's powers as The Crowing begin to emerge and he manages to escape. Later on that priest is killed by Red Army soldiers at Camp Si-Revody. Another Priest, sent to kill Patrick, is successful in it's mission despite Patrick's best efforts to evade and kill it.
One Onstantine Priest pursues Claudio for ten years in Godder Damm. However, Ambellina intercepts it and distracts it each time, sometimes defeating it and sometimes being defeated by it. Eventually it got a chance to attack Claudio directly, but this time Ambellina successfully killed it.
Later on during the Battle of House Atlantic, the remaining Onstantine Priests massacre rebels and Red Army soldiers alike.
The murder of one of the Onstantine Priests forces Mayo to admit to Wilhelm Ryan that he fears Claudio Kilgannon is The Crowing - the one being who could challenge Ryan's rule.
Ambellina and Claudio are met face to face with another Priest, and Claudio viciously attacks it after it injures Ambellina. As Claudio prepares to deal the finishing blow, The Writer intervenes.