An Arco-flagellant is the result of a rare case where a filthy heretic is deemed redeemable enough to be inducted back into the ranks of the Imperium. Of course, such a severe crime is often not left unpunished, and the Ecclesiarchy are sensible enough to punish them in the most appropriate way possible: by turning them into living weapons.
Arco-flagellants occupy a weird ethical and moral ground within /tg/: on one hand the idea of turning convicted criminals into biological weapons would make most modern governments shit themselves... on the other hand, these are traitor heretics we are talking about and sending them to a warzone to redeem themselves of their past wrong doings is quite noble in a way.
See also Penitent Engines, which are the same general concept.
The process of arco-flagellation is a method of penitence administered by the Adeptus Mechanicus, but appointed by the Ecclesiarchy. It is used to punish any heresy not terrible enough to warrant immediate execution. The point of such extravagant punishments is to physically expunge the sin from a heretic's soul so that they may strengthen the Emperor when they die.
In arco-flagellation, the heretic undergoes extensive physical surgery and mental reconditioning, turning them into frenzied berserkers. The guilty person can be implanted with a variety of weapons, such as cutting claws, pneu-mattocks and electro-flails. The Arco-flagellant is fitted with a sealed pacifier helm containing implants that connect into the cerebrum and spinal cord, which keeps them in a calm, barely conscious state. When in this mode the Arco-flagellant's mind is filled with sacred images and verses, such as symbols of the Ecclesiarchy, pictures of Saints and looped hymnals and prayers, which enter directly into the heretic's subconscious. The helm can be deactivated by means of a trigger word, releasing inhibitors and unleashing the full fury of the Arco-flagellant. Another trigger word activates the pacifier helm again, rendering the Arco-flagellant passive once more.
Arco-flagellants are almost unstoppable once activated, as the combat drugs and pain killers shot directly into their spine turns them into roided out maniacs, making them heedless of most injuries they sustain. They are literally Imperial Khorne Berserkers. Arco-flagellants are normally used by the Ecclesiarchy's fighting forces in suicide attack units, because if running blindly into heretical lasgun fire doesn't kill them, the combat stims making to flagellant's body shit itself to death due to the strain of holy combat usually does. However, an Inquisitor can request (or demand if they are confident enough) that a Cardinal turn over an Arco-flagellant or two to their authority. An Arco-flagellant is not a subtle weapon, and they are much appreciated by the more bloody-minded Inquisitors, who are not afraid of some collateral damage and a few innocent casualties.
|Forces of the Sisters of Battle|
|Command:|| Canoness - Ecclesiarchy Battle Conclave |
Ministorum Priest - Sororitas Command Squad
|Troops:|| Avenging Angel - Arco-flagellant - Battle Sister Squad |
Celestians - Crusaders - Death Cult Assassin
Dominion Squad - Retributor Squad - Seraphim Squad
Sister Hospitaler - Sisters Repentia
|Vehicles:||Exorcist - Immolator - Penitent Engine - Repressor - Rhino|
|Flyers:||Avenger Strike Fighter - Drop Pod|
|Saints:||Living Saint - Geminae Superia|
|Forces of the Inquisition|
|Command:|| Inquisitor (Ordo Malleus Inquisitor - Ordo Hereticus Inquisitor |
Ordo Xenos Inquisitor)
|Retinue:|| Acolyte - Arco-flagellant - Astropath - Banisher - Cherubim |
Chiurgeon - Crusaders - Daemonhost - Death Cult Assassin
Jokaero - Hierophant - Mystic - Sanctioned Psyker
Sage (Autosavant - Lexmechanic - Sister Dialogous)
Servo-skull - Servitor - Sister Hospitaler - Pariah - Penitent
Ministorum Priest - Militarum Veteran Squad - Warp-Seer
|Auxiliaries:|| Inquisitorial Stormtroopers - Deathwatch - Grey Knights |
Sisters of Battle - Callidus Assassin - Culexus Assassin
Eversor Assassin - Vindicare Assassin
|Vehicles:|| Chimera - Land Raider (Land Raider Crusader |
Land Raider Redeemer) - Razorback - Rhino