A Diabolist was a unique officer class utilised exclusively by the traitorous Word Bearers Legion during the Horus Heresy in the early 31st Millennium. The term, "Diabolist", was first coined by the few survivors of the 22nd Chapter of the Ultramarines Legion during the Battle of Calth in 007.M31. This designation was later adopted en masse by their brethren to identify those Word Bearers commanders whose training appeared to somehow harness the Ætheric medium as a weapon of war. These Consul-type officers represented a Word Bearers officer who had already become engulfed in the unholy lore of Chaos and had paid the forfeit with their soul. They were able to direct the corporeal manifestations of the Warp -- manifesting what some observers have called "Warp magic". Carefully hidden from Ultramarines observers, other officers of similar specialisation to those observed in various cities appeared to have been present in most of the Word Bearers units mustered on Calth. Indeed, ritual use of psychic powers by these individuals appears to have been the source of some of the bizarre transformations and illogical tactical choices made by certain Word Bearers formations. One of many dictates issued by the Primarch Roboute Guilliman in the campaigns following Calth was the priority targetting of enemy officers classified as Diabolists. These specialist officers are akin to the present-day Word Bearers' spiritual leaders, known as Dark Apostles. It is not known if these individuals are still utilised in the ranks of the Word Bearers Traitor Legion in the present era of the late 41st Millennium or have been completely subsumed by the Dark Apostles.
- The Horus Heresy - Book Five: Tempest, by Alan Bligh, pp. 113, 158, 242-243