Malcador, called the Sigillite, has been in the Emperor’s court since before Unity was achieved, and served as the Emperor’s closest advisor during unification.
Malcador founded the Administratum of Terra, which handled the day-to-day runnings of the fledgeling Imperium, and often worked as the Emperor’s proxy in those days and afterward, allowing the Emperor to continue working on private interests such as the creation of the Primarchs. To stand in the presence of the Sigilite was to be under the eye of the Emperor Himself.
Malcador was an extraordinarily potent psyker, second only to the Master of Mankind and perhaps, later, his son Magnus. However, the sharpness of his mind was turned most often toward other tasks. As the Heresy unfolded, Malcador became the Master of Assassins, standing at the head of the Officio Assassinorum, and used his influence to order several attempts on the traitor Warmaster’s life. On the Emperor’s orders, Malcador began to recruit men and women of character, skill, and determination to be His investigators in these uncertain times. The individuals Malcador chose became the first elements of the latter-day Inquisition; a subset known as his Knights-Errant would grow into the Grey Knights, chamber militant of the Ordo Malleus subsect of the Inquisition. He also aided Rogal Dorn in the preparations of the defense of Terra.
During the siege of the Palace, as the Emperor prepared to board Horus’ flagship, Malcador was given the seat of the Golden Throne, which helped create the psychic sheath that prevented daemons pouring from the ruined human Webway. Magnus had originally been chosen as the steward of the Throne, but the turn of events had eliminated that possibility. The effort of holding the fabric of reality together nearly killed the Sigilite, and as he was disengaged from the throne that the Emperor might be interred there, he died, passing the last spark of his life to his liege lord. The Emperor was able to awake from his coma long enough to give his final orders to his servants because of Malcador’s sacrifice.
When I say “Malcador is just a normal dude,” I mean that as a compliment, because in an age of giants and gods, a mortal psyker, no more than an average man by most measures, stands head and shoulders above many of his peers.