From Lexicanum

Malcador the Sigillite was the First Lord of Terra during the Horus Heresy. He was a powerful psyker who could communicate over long distances. He was a close advisor to the Emperor during the Battles of Unification, and the man credited with founding the Administratum of Terra

During the final chapter of the Heresy, the Emperor told Malcador that he had to take his place on the Golden Throne - providing the psychic sheath to protect the human-built sections of the Webway. His original choice of replacement on the throne had been the Primarch Magnus but since he had sided with Horus, Malcador was now his chosen successor. He is noted for having given the Chaplain Edict at (presumably) Nikaea, a group of Marines that would maintain the order and discipline of their fellow Marines, and to keep watch for heresy and psychic powers, the former to be dealt with summarily, the latter to be referred to the Legion Librarium.

In the days before the final confrontation between the Emperor and Horus, the Emperor told Malcador to summon "men of character, skill and determination" who would be tested and trained to become an elite group of investigators with the task of rooting out treachery and heresy. This marked the founding of the Imperial Inquisition. Among the first Malcador approached for this duty were Death Guard hero Nathaniel Garro and former Sons of Horus Marine Captain Iacton Qruze¹.

Though he was a powerful psyker in his own right, Malcador's mental powers were nothing compared to that of the Emperor, and so when Rogal Dorn brought the severely injured ruler of mankind back to the Golden Throne, he found Malcador sitting wasted, energy lashing across his shriveled body, tortured by the psychic bombardments of the webway. He was almost dead. Techpriests then made the exchange - disengaging Malcador from the machine. As he was removed, the last flicker of life left him and the dust of his corpse blew across the stone floor.

At the instant of Malcador's death, the Emperor awoke from his coma - Malcador had saved a last kernel of strength and passed it on to the Emperor so he could give his servants their final orders.²

Sources

• ¹:Flight of the Eisenstein by James Swallow.
• ²:Horus Heresy: Visions of Death by Alan Merrett