Chosen of Malcador was a secret army, sent to the far reaches of the Imperium of Man to prosecute the Sigillite's "Silent War" during the Horus Heresy.

The so-called "Chosen of Malcador" included many individuals from a wide range of backgrounds, but his Knights-Errant were all Space Marines, and many of these were the scions of the Traitor Primarchs who had refused to follow their gene-fathers into treachery and chose instead to renounce their Legion and serve the Imperium directly at its highest level. All cast off their original Space Marine Legion heraldry and wore in its stead the gunmetal grey of bare ceramite.

Calth (007.M31)
"You court death, Garro, like some ancient Knight-Errant."
--"That's as good a title as any, Brother."