Exorcist
Not to be confused with the Exorcists Chapter of the Space Marines.
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 higher 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).
There are two main patterns of Exorcists in existence. The most familiar is the Prioris pattern, with a missile-launching pipe organ mounted on the back. Forge World released a later model, the Sanctorum pattern, with a more sensible-looking, but vastly less awesome, missile launcher in the shape of two twelve-rocket launch pods.