The Exorcist is the main artillery unit employed by the Sisters of Battle. It is similar in construction to the Space Marines' Whirlwind Artillery Tank, in that it is a Rhino body with a missile launcher on top, though where the Whirlwind fires one missile with a large blast radius, the Exorcist fires many smaller missiles of lower strength but more concentrated effect.
At least, that's the goal. Unfortunately, the Exorcist is a very old pattern, dating to the Age of Apostasy (in fact, some of the tanks constructed then are still in service), and the Ecclesiarchy is of the opinion that it would be heresy to allow a less holy branch of the Imperium to use these vehicles of divine retribution. As such, the Adeptus Mechanicus (whose relations with the Ecclesiarchy are strained at best) only gets to service the Exorcists very infrequently, so the machine spirits are a little unstable. An Exorcist may fire six missiles in a single salvo, or just one, or any number in between (as the launcher's type is Heavy d6).
This was buffed in 8th edition, where Guilliman apparently browbeat the Sororitas into bringing their vehicles in for a tune-up, and now they fire 3d3 missiles.
There are two main patterns of Exorcists in existence. The most familiar is the Prioris pattern that first appeared in the Witch Hunters codex, with a missile-launching pipe organ mounted on the back. A year before, Forge World had released the Sanctorum pattern to go with the early 3rd edition Chapter Approved codex. As the Exorcist had absolutely zero fluff or art at the time, they took the easy route and made it a Whirlwind expy sporting two twelve-rocket launch pods, a more sensible design but vastly less awesome than the VLS organ.
Ironically, not building the Exorcist for the Emperor's armies breaches the Treaty of Olympus. The Ministorum is not part of the military. As such, restricting the Exorcist's (and other vehicles) use to the Ministorum is both heresy and treason by their own laws. Of course breaking laws like this is pretty standard for the Imperium. Also don't tell the Sisters that they're breaking any laws. It can, and will, result in being purged.
If you're wondering how a pipe organ on a Rhino works like a tank, let me educate you. The exorcist is basically a MLRS (Or Multiple Launch Rocket System for those who do not know) top-attack ATGM (Anti-Tank Guided Missile) in a VLS (Vertical Launch System) configuration; as you can see the military loves acronyms. That pipe organ on the back of this Rhino, that's that rocket system that fires the rockets. The fire control, the keyboard. Also, while this might make you think its an artillery piece, that's not correct. The rockets the Exorcist fires are meant to bust vehicles, not masses of infantry. This is why the exorcist launcher does not use a blast template and why it looks like a freakin' pipe organ. The rockets would have to go straight up, arch downward and strike the thinnest armor on the vehicle, the top of it. Basically the same as a modern-day Javelin missile. That being said, if the roll is high enough, it can destroy a massive amount of infantry in one salvo because nobody likes being brained by meter-long missiles. For a real life equivalent, get a look at the russian 9K330 Tor SAM, same basic design even if it kills planes and not tanks.
|Forces of the Sisters of Battle|
|Command:|| Triumph of Saint Katherine - Canoness |
Ecclesiarchy Battle Conclave - Imagifier
Ministorum Priest - Sororitas Command Squad
|Troops:|| Avenging Angel - Arco-flagellant - Battle Sister Squad |
Celestians - Crusaders - Death Cult Assassin
Dominion Squad - Retributor Squad - Seraphim Squad
Sisters Repentia - Zephyrim Squad
|Vehicles:|| Exorcist - Immolator - Penitent Engine - Repressor |
Rhino - Mortifier
|Special Vehicles:||Pulpit of Saint Holline's Basilica|
|Flyers:||Avenger Strike Fighter - Drop Pod|
|Saints:||Living Saint - Geminae Superia|
|Non Militant||Orders Dialogous - Orders Famulous - Orders Hospitaler|