The Banishers chapter was founded as part of the 13th Founding in M36 alongside the Exorcists, with similar purposes and methods. As part of their training they invited possession by minor warp entities under controlled conditions to enable them to better resist such intrusions.
For most of their history the chapter had a distinguished record against all manner of foes. Over time their careful safeguards began to fail, opening them to corruption. Their contact with the denizens of the warp left residues of foreknowledge that aided them in future battles, and such success left many hungry for further insight. Under the supervision of the Binders – the chapter’s Apothecaries and Librarians combined – marines sought longer and longer possessions by more and more powerful entities, both as a measure of pride and to wrest knowledge and power from them.
The Binders labored along two tracks to perfect the chapter’s possession methods – physically improved recruits better able to resist the corporeal rigors, and better psychic measures to allow for domination of the daemons rather than the other way around. The chief of the Binders in M38, Shaitan, was especially driven to push the envelope, and through his guidance the chapter acquired more targeted knowledge and equipment, sometimes through intentionally false readings of the Tarot leading to attacks on isolated Mechanicum installations and the purging of a “rogue” Inquisitor, with the confiscation of her library and artifacts.
Over time as such events became more common and the Binders’ experiments took ever more esoteric turns, others within the chapter leadership began to have more doubts as to Shaitan’s sincerity. Chapter Master Hezekiah, accompanied by several Chaplains, burst into the concealed lower levels of the Binders’ laboratories and uncovered secrets that horrified them. Cloning operations, forbidden geneseed tampering, and confined subjects of possession that had been allowed to run its course so that the changes over time could be studied closely.
Enraged at how far they had strayed, Hezekiah demanded that Shaitan and the Binders destroy the possessed marines and end their heretical studies at once. Unwilling to be denied, Shaitan refused and battle soon erupted with the possessed being freed, slaying the Chapter Master and Chaplains. Once loosed, the bloodletting would not be easily caged again. Partisans of the Binders fell into line and clashed with those who doubted the more extreme paths the chapter was pursuing. War raged within the corridors of the Fortress-Monastery, with brother slaying brother as previously concealed sorceries lashed out and previously tightly controlled daemons were given free reign against the reticent.
Shaitan’s faction emerged triumphant, but not without great loss. Acclaimed master of the reduced chapter, Shaitan began to lay the next stage of plans as the Binders carefully collected the geneseed of their fallen brothers, friend and foe alike.
(This is the first part of the background for the eventual reworking of my Chaos Marines. More will be forthcoming over time, but models will likely wait until after I get the Space Sharks sorted. As a note, doing traitor background is generally more involved, as you have to come up with how they were before the turn, then why they turned, and what they're like now. Loyalists have it so easy!)