The Immolator is the Adepta Sororitas' standard Infantry Fighting Vehicle, and their answer to the Space Marines' Razorback. The main difference is that the Immolator's turret is mounted towards the front of the vehicle, rather than the back.
The Ecclesiarchy actually has a monopoly on this style of assault tank, as a result of the circumstances of its discovery. Its STC pattern was found before the Age of Apostasy, when the Ecclesiarchy was still allowed to have men under arms; said men, the Frateris Templar, discovered an ancient factory on one of the worlds they had reclaimed for the Imperium. They decided to keep it a secret, to use as a bargaining chip later.
They would use that chip shortly after the Age of Apostasy; with the Frateris Templar was disbanded, all of their vehicles went with them, and even though the Ecclesiarchy was able to rules lawyer its way into keeping the Sisters of Battle (being women under arms), they still needed to be armed somehow. The Ecclesiarchy arranged a deal with the Adeptus Mechanicus: they would give the data from the factory over to the tech-priests, in exchange for an exclusivity contract on any equipment produced from it. Deciding that access to ancient data was more important than open license agreements, the Mechanicus agreed, and the result, when the holes were patched with the Rhino STC plans, was a heavy flamer-toting assault tank (though later patterns substituted the heavy flamers for heavy bolters and multi-meltas). This act is a violation of the Treaty of Olympus by both parties. As such, it 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.
A sad moment of note may have come to pass however, for as of November/December 2015, the Immolator is no longer available from the GW website. As this happened the day after I ordered one, it makes me think that I might have bought the last one ever... Just as planned.
however, there is still hope, as the exorcist kit now includes the equipment to make an immolator - according to a GW staff member who plays sisters (confirmed - I, a simple fa/tg/uy, recently bought an exorcist in lieu of an immolator and was pleasantly surprised to find all the goodies for an immolator included in the kit. The glass canopy is no longer there though, instead its just this plastic ring piece, but it does the job. Not sure why the website doesn't include the fact that the kit doubles as an immolator.)
As of Dec.14, 2016, the immolator is back up on GW's web store. That's right, they're back, motherfuckers! After another absence they officially returned to the GW online store in September 2017 giving you so much firey dakka it'd make an Ork shed a tear
|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|