The Knights Errant were a secretive company of Space Marines gathered by Malcador the Sigillite during the darkest days of the war, drawn one by one from their parent Legions to serve the Emperor directly. Stripped of old allegiances and clad in unmarked armour, they became agents of the Regent of Terra, dispatched on missions too vital or too secret to entrust to any Legion.
Malcador chose them for more than martial skill. Each was a warrior who had, in some manner, slipped the bonds of his Legion's fate — survivors, exiles, and men touched by destiny, plucked from the chaos of the Age of Darkness to labour in the shadows. They hunted traitors, recovered lost knowledge, and undertook errands upon which the survival of the Imperium might quietly turn.
The Knights Errant were a seed planted for the future. From this hidden brotherhood would grow the institutions that guarded the Imperium in the millennia after the war — the ordos that watched for heresy and the silent orders that fought the daemon. In an age when the Legions had proved that even the mightiest could fall, Malcador sought a different kind of servant: loyal not to a primarch, but to Terra alone. See the Horus Heresy explained.