The Ordo Malleus is the oldest arm of the Inquisition, created by decree of the Emperor prior to his internment in and ascension to the Golden Throne of Terra. In the hours following the defeat of Warmaster Horus, and understanding the severity of his own wounds, the Emperor believed that others must fill his role guarding humanity from the Warp, to thwart the designs of the Chaos Gods.
"They shall be My Hammer, the sword in My hand, the gauntlet about My fist, the bane of My foes and woes of the treacherous. When no others may stand beside them, they shall fight. Only the greatest shall enter their ranks, for unto them do I entrust stewardship over the Gates of Hell." Thus Spoke the Emperor.
Historians and savants point to this declaration, known as the Charge of the Malleus, as the Ordo's founding moment. The Charge is written in gold over the Ultimate Gateway of the Malleus Fortress on Titan, and is reaffirmed by every Malleus member upon elevation to the rank of inquisitor.
It is known that during the Great Crusade, the forces of the Emperor encountered many worlds that worshipped foul daemonic beings as gods. Such worlds were either cleansed or cast into the warp though those that were not were instead had their daemons imprisoned by psychic wards on that planet. Following this task, the Ordo Malleus created the Brotherhood of the Watch to guard these prison worlds.
The power and authority of the Ordo Malleus extends beyond the nominal boundaries of the Inquisition, a jurisdictional extension known as the Malleus Remit. In events of grave emergency concerning potential daemonic threats, a Malleus Inquisitor may demand the services and resources of any Imperial servant or organization, a request which may not be refused on pain of death. Even the High Lords of Terra and the Adeptus Astartes are beholden to follow the Malleus Remit without hesitation.
The Grey Knights
The Grey Knights are the legendary Chapter 666 - and although nominally a Chapter of the Astartes, they are in fact part of the Inquisition's Ordo Malleus, serving as the ordo's elite military forces. They were founded during the Second Founding, and are rumoured to have the most pure of all gene-stock. Each member of the chapter undergoes a gruelling and torturous selection process, and even once inducted, their harsh training regime is without equal. In battle, they move as an army of silver ghosts, surrounded by awe, and equipped to the teeth to deal with the worst foes that Chaos can raise to meet them.
The Grey Knights hold a unique honour among the chapters of the Adeptus Astartes - in over ten thousand years of service, not a single one of their number has ever defected to Chaos. Their armour is left unpainted, leaving the silver-grey of the ceramite exposed. This tradition is thought to have originated from their desire to lead lives of absolute purity. Upon achieving the rank of Justicar one may display their personal heraldry on a shield on their shoulder. In contrast to this their armour is highly decorated, encrusted with protective symbols and engraved litanies5.
Just before the Second Founding, the Imperium was still reeling in shock from the recent galactic civil war, and very vulnerable. The Emperor ordered the creation of a 'secret chapter' of the Space Marines: chapter six-hundred-and-sixty-six. The gene-seed for this chapter is strongly suspected to have been taken from the Emperor directly. After their creation, the chapter was assigned to the Ordo Malleus as their Chamber Militant. The name of this chapter has become legend, and is feared throughout the galaxy by those it opposes, and is whispered in reverence by all those it protects: the Grey Knights.