The Absolvers are a fleet based chapter from the 17th Founding. They were created with strong ties to the Inquisition, and they serve as a reserve force that may be called upon by any Deatchwatch teams or Ordo Xenos Inquisitors. They are generally only summoned for actions that require company to chapter sized element support. Most commonly, this is to address alien threats too large or difficult to be handled by the squads customarily utilized by the Ordo Xenos.
As this position is somewhat unique within the Imperium, the geneseed of the Chapter is shrouded in secrecy, although it is regularly tested for purity. The Absolvers act as a large scale force for the Ordo Xenos but possess less autonomy than a normal Space Marine chapter. They are a silent and loyal force capable of working alongside or destroying xenos as needed.
The Absolvers chapter itself is comprised of somewhere between 1,200 and 1,500 marines with no scout company. Most of these are relatively inexperienced, and psycho-indoctrination takes the place of lengthy training in a Scout Company. The Chapter will readily enter into battles of attrition and accept the losses these entail. The best most Absolvers can ever hope for their future is an anonymous death along with their battle-brothers. Those that survive for more than a century of service will join the small cadre of veterans that coordinates the bulk of the tactical and strategic workings of the chapter.
If a marine survives the first 100 years of his service to the Ordo and the Emperor, he may be promoted the rank of Hetairoi. At this time, he is fit to lead Absolvers in combat. The toughest and most ferocious of the Absolvers alone live to see this honor. Only upon attaining the rank of Hetairoi can an Absolver aspire toward specialty positions such as techmarine, chaplain, and librarian. Command positions are rotated thrice annually amongst the Hetairoi within a battle group, or within a theater should a conflict continue long enough. This practice was implemented to prevent one faction within the Ordo Xenos or Deathwatch from being able to control the entire Absolvers chapter by catching the ear of the Chapter Master.
I hope this brief look at the Absolvers inspires you the reader to take a good look at your fluff and see where you can let your imagination run wild.