Not every servant of the Thousand Sons is a dust-filled suit of armour or an immortal sorcerer of the Legion. Across countless worlds the sons of Magnus cultivate hidden cults among the mortal population, drawing the ambitious and the desperate into the worship of Tzeentch, and at the head of each such coven stands a Magister. These are men and women of flesh and blood, elevated above their fellow cultists by cunning, by devotion, and by the fragments of true sorcery their distant masters see fit to grant them.
A Magister is a figure of considerable power in his own right, a mortal sorcerer-lord who commands both the braying ranks of the Tzaangor beastmen and the human covens from which he rose. He channels the energies of the Warp much as the Legion's Astartes do, if with lesser mastery, and turns that stolen craft to smiting his enemies and binding his followers ever tighter to the Architect of Fate. The very title echoes the Legion's own hierarchy, in which the leaders of the great sorcerous cults bore the rank of Magister Templi.
Such champions serve the Thousand Sons as agents and vanguard, preparing worlds for the coming of their masters and shedding blood in the Changer's name where no Rubricae yet march. To rise to the office is the highest honour a mortal follower may attain, and the surest sign that Tzeentch has taken notice of him, a favour that proves as often the prelude to some greater and more terrible transformation as it does a reward.