May I present to you my humble Space Marine army:
The Divine Exorcists
The Divine Exorcists were once clad in red armour, as part of the Exorcists chapter, so are very similar to the Exorcists, in that they are trained by Grey Knights, enabling them to focus their time on fighting chaos and the daemonic. However, at some point during the 38th Millenium, arguments erupted between the various companies over codes of practice. Most of the 3rd 5th and 10th companies, many members of the (small) Veteran Company, some administrative staff and and a few Librarians believed that Xenobiologist Lauram Clelland’s process of exposing recruits to the essence of the daemon was a heretical idea that only resulted in the later daemonisation of the recruits. They divided from the rest of the chapter and repainted their armour in colours that contrasted with those of the Exorcists. Having been trained in the same way, the rebels, under the direction of Ordo Malleus
Inquisitor Lord Mercurius, conducted new rituals to purify and cleanse their souls, ready to form a new chapter.
They sought the leadership of Inquisitor Mercurius, but he was too engrossed in his own toils, hunting down daemons, and had a different agenda to the new chapter. They then turned to the Librarian Arcturus, who was one of the only authoritative members of the Exorcists who rebelled. It is he who became their equivalent of Chapter Master. He gave himself the title of Grand Librarian, and decreed that the chapter would be led by librarians. He then assigned a librarian to lead each of the 3 newly formed companies.
New recruits to the Divine Exorcists are
not exposed to the essence of the daemon, and are instead trained using the knowledge that has been gathered over the millenia by Mercurius and the Librarians of the chapter. This information gives the chapter a unique slant on the principles of the Codex Astartes.
It is generally the Space Marines who call on Mercurius, as he prefers the support of Grey Knights. However, he is dedicated to hunting daemons, and so rarely lets the Divine Exorcists fight on their own if he can help it, and always brings his personal bodyguard of servitors, acolytes and inquisitorial stormtroopers, who make for a formidable and versatile force on their own.
The Divine Exorcists are by no means a standard Codex Chapter, and in fact have a notable divergence from the Codex Astartes, shown in their ability to use their bolters in one hand, enabling them to arm themselves with exquisite curved swords, not too dissimilar from those of the World Eaters, to use in close combat. This is, of course, a huge asset when fighting daemons, who nearly always prefer close assaults. They also have a distinct lack of heavy tanks and transport vehicles, preferring to rely on stealth to take them behind enemy lines. These abilities were developed specially for combating the daemonic, who, being creatures that do little thinking, are easily disrupted by space marines appearing from the shadows.
Though the Divine Exorcists may have a notable divergence from the Codex Astartes, they do try and follow the chapter organisation rules. However, it has only been 4,000 years and there is a distinct lack of suitable candidates for recruitment on their home planet of Alpharmenius. This has made it difficult for them to maintain a full 10 companies of marines. The librarians are set the difficult task of searching neighbouring planets for recruits amongst the billions of people that inhabit them.
~Ice~ 
