Here is a concept I have had banging around inside of my head but has never been realised due to lack of disposable income and university work, but here goes.
After the death of Horus his legion was conflicted as to their leadership. When Abaddon declared that Horus was dead and lead the legion into the Eye of Terror there were those that disagreed. One of the Captains of the legions believed that it was Horus, not the Emperor who was the true god of Mankind, and as a God he could not be fully destroyed. His mortal vestiges had been destroyed but his spirit had been released into the universe, His advisors had even unearthed an ancient prophecy which stated that the god of mankind would be reincarnated and return to lead his people to greatness once again.
Gathering together what support he could, he took those loyal to him and to his ideals with him, scouring the galaxy for the new vessel of the Spirit of their God, Horus. They would search many worlds, and engage in many conflicts, but would remain pure and relatively unblemished by the forces of chaos. When those who started the search started to grow old and die, they realised that they would need others to continue their work and so their apothecaries outfitted their ship, the "Spirit of Horus" with the medical facilities needed to create a next generation of Sons of Horus to continue their search, and they dispatched their most persuasive and covert members to find and recruit suitable candidates.
Forces - Horus Heresy Period Luna wolves/Sons of Horus, with their armour repaired with whatever resources they could scrounge together in their search. Perhaps Cultist support as well
Army - Either Space Marines to get the organisation, or Choas space marines and treat the marks as certain members leaning into the various blessings of "Their God" e.g. Mark of Khorne - Veteran Assault marines who think that Horus has Blessed them for their ferocity.
Thats about it really, just a bunch of Sons of Horus searching for the reincarnation of the man they think is a god