Exorcists

Exorcists
Battle Cry	Chanting in a mixture of dark languages and high gothic
Number	Unknown
Founding	13th Founding
Successors of	Grey Knights (unconfirmed)
Successor Chapters	None
Chapter Master	Unknown
Primarch	Unknown, possibly the Emperor
Homeworld	Banish
Strength	1200 Marines
Specialty	Fighting Daemons(Mostly lesser Daemons as Grey Knights are too valuable)
Allegiance	Imperium
Colours	Word Bearers shade of Red and Pure Black

The Exorcists are a Space Marine Chapter founded during the Dark Founding and is one of only two canon Chapters known to have come from that Founding (the other being the Death Spectres). Their shtick is basically "reasonable Grey Knights." Seriously, check out the table below.

Grey Knights	Exorcists
Recruit from Psykers, who are pretty much Daemon-chow.	Recruits are possessed and exorcised of Daemons, making them invisible to Daemons.
Kills or mindwipes everyone who sees them	Don't murder innocent civilians for being saved by them
Recruits from an already rare population and only 1 in a billion survive	Recruits from an entire planet, and two extra scout companies offset the lethality of Daemonic Possession Therapy
Only fights Daemons	Actually fights non-Chaos enemies on a regular basis
Secret base (Titan, moon of Saturn) is in one of the most overpopulated planetary systems in the Imperium (though mitigating the stupidity, this puts them a short jump away from the warp gate under the Golden Throne if it flares up beyond what even the Adeptus Custodes can handle)	Secret base (Banish) is in the middle of a forbidden sector

They have ties to certain factions in the Ordo Malleus; it's never been mentioned which ones, but they're almost definitely Radical. Although they largely follow the Codex Astartes, they also have two extra Scout companies (exorcism is a haphazard process, after all), and they have special cross-Chapter organizations called Orisons. The Orisons invite exceptional warriors from across the Chapter to study a particular style of combat. The Chapter also doesn't like to stick to any one tactic for very long; indeed, their Chapter Tactics allow you to choose a different Chapter's Tactics before every battle! They also have an exceptionally large fleet (including three Battle Barges), which allows them to respond (relatively) quickly to any emergent threat and remain on extended operations much longer than other Chapters. Technically, the Exorcists recruits are not exorcised of daemons. The recruit must kick the daemon out of himself through sheer willpower. Otherwise he is killed by the other Astartes.

Funnily enough, with their color scheme of red, black and bone; use of geometric symbols and runic script on their armor; use of demonic possession; and use of dark chanting, they're basically loyalist Word Bearers.

Notable Characters

• Silas Alberec: The Brother-Captain of the Third Company, he is really fucking huge, like Ogryn or maybe even Primarch sized. He is slated to be the next Chapter Master. He wields an artifact mace known as the Hellslayer, which is made of a material so heavy that only his brutish strength can lift it. He led the Exorcist forces in the Badab War. He is also the only in-game playable character for the exorcists, and he (awesomely) auto-wounds daemons at S10 ap2 master crafted. Hilariously, he can bring more pain against daemons than Kaldor Draigo could ever hope to.
• Enoch Trismegistus: The first Chapter Master of the Exorcists. The Orision known as the Enochian Guard is named after him. With two companies, had a 97 to 1 kill rate, with only 1% of the Exorcists getting recorrupted, so two marines total. Also his surname is taken from a real life semi-legendary alchemist and means Thrice the Greatest.
• Darrion Rauth: A member of the Deathwatch from the short stories Headhunted and Exhumed (written by Steve Parker). An asshole who really hates psykers, though he goes from "asshole" to "creepy" in the prequel novel Deathwatch.
• Waite Ryder: Master of the Fleet. Saved the life of Carab Culln, Chapter Master of the Red Scorpions.