Warhammer: The Horus Heresy: Liber Imperium Emperor Army Book

Outside the loyalist Space Marine Legions and the Cult Mechanicum, the Emperor wields forces which are loyal only to him. The Legio Custodes guard the Emperor's person, while the Sisters of Silence stand watch over the Black Ships.The elite spearhead of the Imperial Army, known as the Solar Auxilia, exemplifies humanity in the Emperor's name, while the Divisio Assassinorum is his culling blade, aimed at the neck of any foolish enough to imperil his great dream of unity.