Capture Company Argus

Founding Lost to Time - "This Company has been around since the early days of the Great Crusade, perhaps even being created alongside the Custodes themselves, alternatively it may have been redacted intentionally." Shadowkeepers so redacted lol.

Warband purpose Search and Recovery - It could be finding relics from the Dark Age or recovering the ring of a wealthy noble.

Why the captain was promoted In his wisdom and charisma, the captain-commander managed to sway a traitor commander to surrender despite being surrounded on all fronts and massively outnumbered, simultaneously ending the conflict with minimal damage and helping the traitors find redemption (in suicide attacks after brainwashing, of course, though their souls are hopefully fine).

Combat Prowess/doctrine Without Flaw: These Custodes either engage in melee or fight from range, but either way do so in a way that most battles are fought without a scratch. Aid Unlooked For: The Host arrives from the blackness of space to claim victory against a foe, be it mortal or malign, and then depart into that same blackness as arrived.

Reputation Hardly Heard: Perhaps suffering from what others would look upon as an ignoble duty, guarding a sancturary or standing watch on a moon, this Host is unheard of to most of the Galaxy besides the most well versed.

Allies Makes use of the Guard of the Imperial Household of Terra

Great Enemy A particular Daemon, Daemon Prince or Disciple of Chaos