The Cults of the Thousand Sons represented sub-formations composed of potent psykers within the Thousand Sons Space Marine Legion who specialised in certain psychic and sorcerous disciplines during the Great Crusade and Horus Heresy eras. The Thousand Sons, like some of their fellow independent-minded Astartes Legions, employed a non-standard organisation within the XV Legion that were known as Cults and into which every Astartes was placed based upon their level of psychic ability.
Though some Thousand Sons Astartes never developed any measurable amount of psychic ability after being implanted with the XV Legion's gene-seed, the majority did develop some form of psychic ability just like their Primarch Magnus the Red soon after completing the transformation process. The Legion used these five different Cults to develop and improve the abilities of the often powerful psykers found amidst this Legion's ranks. Membership within these Cults was determined regardless of the Fellowship (Chapter) to which each Astartes belonged within the Legion since Cult membership was based upon the psychic ability for which a Thousand Sons Astartes displayed the most affinity. Each Fellowship of the Thousand Sons was usually made up of Astartes coming from all five of the Cults within the XV Legion, though it was not unheard of for a Fellowship's membership to be dominated by the members of just one of the Cults.
These five Cults had been founded within the Legion by the nine original members of Magnus the Red's inner circle or cabal who were collectively known as the Rehati. The Rehati had each earned mastery in one of the 5 recognised psychic disciplines. Each Cult was formed to further the development of one of the recognised psychic aptitudes: the Athanaeans focused on telepathy, the Corvidae sought to develop their precognitive abilities, the Pavoni learned to make use of physiokinesis, the mental manipulation of one's own physiological processes, the Pyrae plumbed the secrets of fire, a discipline called pyrokinesis, and the Raptora mastered the potent power of telekinesis. The leader of each Cult bore the rank of Magister Templi. The leader of all the Cults was known as the Magus and this was a rank exclusively held by the XV Legion's Primarch, Magnus the Red. The highest rank within the Cult system was that of Ipissumus, a grade which represented a being free from limitations, who lives in balance with the corporeal and incorporeal universe. For all intents and purposes, the Ipisssumus was the most perfect being in the universe. This esteemed rank was represented by the most powerful human psyker in existence -- the Emperor of Mankind.
On the Thousand Sons' lost homeworld of Prospero, in the capital city of Tizca, each Cult maintained its own pyramidal headquarters, which served as both a repository of the arcane knowledge that they had collected about their chosen psychic disciplines from across the galaxy as well as training facilities specifically geared towards enhancing psykers who displayed strength in one of the five recognised psychic disciplines. Each of these pyramids' front gates was topped by the great golden icon that represented the Cult, save for the pyramid of the Pyrae Cult, which was topped by an ever-burning flame and whose gate was guarded by Canis Vertex, a Warlord-class Battle Titan of the Legio Astorum that would see action during the Burning of Prospero.
To further represent the distinctiveness of the Cults' members, the Astartes from each Cult would display the Cult's insignia upon the shoulder plates of their Power Armour, with the Cult's distinctive icon placed within the centre of the XV Legion's Pre-Heresy Legion badge.
It is currently unknown what happened to the Cults of the Thousand Sons Legion in the aftermath of the Horus Heresy and their transformation into a Traitor Legion of Tzeentch, as well as the vast changes wrought in the XV Legion by the Rubric of Ahriman. It can be assumed that the same range of basic psychic powers not only still exists among the Thousand Sons but has probably been greatly enhanced by their residence within the Eye of Terror and their innate connection to the Lord of Change. The five known cults of the Pre-Heresy Thousand Sons Legion are described below.
The Athanaean Cult's members were practiced masters of the discipline of telepathy, and were able to transmit their thoughts and read the thoughts of others. They often acted as the Legion's communications officers since their communications were secure from all but other psykers and they assisted the Corvidae in conducting intelligence operations based on psychically-gleaned knowledge. While the Corvidae could provide an overall strategic direction based on their reading of the likely probabilities of future events occurring, the Athanaeans could use their ability to read the minds of the enemy to shape the Legion's battlefield tactics more directly, through weapon choice, the types of troops to be deployed and where, and the choice of the most advantageous terrain.
The Corvidae were skilled in the psychic discipline of precognition, determining the likely probabilities of future events. They served as the XV Legion's seers, warning their Battle-Brothers of dangers before they materialised. The Corvidae Cult also helped guide the Thousand Sons Legion along the lines of Fate during times of conflict and in pursuit of their overall psychic and material growth as a Legion. The Corvidae were responsible for much of the Legion's strategic planning. Corvidae could determine events that were not only far in the future but were immediately about to happen, which allowed them, for example, to navigate a minefield before it exploded or to know the exact sequence in which a series of enemy soldiers would fire at a particular target. The Cult badge of the Corvidae was a black raven's head.
The Pavoni Cult tended to be the most egocentric of the known Cults within the Thousand Sons Legion, as they had developed the psychic discipline known as physiokinesis. Physiokinesis allowed the Pavoni to manipulate body chemistry and physiology within themselves as well as others, a very useful trait for a group of already superhuman warriors. Skilled practitioners of physiokinesis were able to alter their physical appearance by manipulating the physical characteristics of their facial features at will as well as making themselves immune to the harmful effects of dehydration and other physical ailments. On the field of battle, the Pavoni were able to generate lightning by unleashing the inherent bioelectricity of their own nervous systems and focus it offensively, as well as cause the hearts of enemy warriors to explode by literally boiling the blood within their veins. When using these abilities against their foes, the Pavoni had a reputation for venality and spite which bordered on the sickening. The Pavoni were also the designated healers of the Legion, as they were capable of accelerating cellular growth and protein synthesis, which made them extraordinary medics who saved the lives of countless Astartes when even the Apothecaries could not.
The Pyrae were pyrokinetics, meaning that they had developed the ability to both generate and control fire, the power for which was drawn from the Warp. The Pyrae could both generate lethal bursts of fire from their bodies that they could shape into a number of different forms, including projectiles fired from their hands as well as fire shields that could hold back foes and melt away incoming weapons fire. The Pyrae could manipulate existing or standing sources of fire and with their connection to the Warp could lower the temperature of flames striking their own bodies or the bodies of others, effectively making them immune to the effects of fire or great heat. The Pyrae also developed secondary psychic abilities that allowed them to control and manipulate mechanical automata like the robots of the Legio Cybernetica and even the massive Titans of the Collegia Titanica. These secondary abilities were known as technopathy. At the time of the closing years of the Great Crusade, the Pyrae Cult was in the ascendence within the XV Legion's hierarchy, while the Corvidae Cult was at its lowest ebb for nearly fifty standard years. For centuries, the Corvidae had been pre-eminent within the ranks of the Thousand Sons, but in the Legion's final decades within the Imperium, their power to read the twisting paths of the future had diminished until their seers could barely penetrate the shallows of things to come. The Cult badge of the Pyrae was a phoenix wreathed in flames.
The members of the Raptora Cult were gifted practitioners of the psychic discipline called telekinesis, and were known as telekines. In battle, a Raptora Cult member could psychically fling physical objects at their enemies or rip their enemies bodies' apart through the application of immense telekinetic power drawn from the Immaterium. When used defensively, telekines could generate barriers of invisible kinetic force known as "Kine-Shields" to block physical attacks on themselves or others. During the Burning of Prospero, the massed members of the Raptora Cult generated a massive kine shield over the whole of the city of Tizca intended to protect it from an orbital assault by the Space Wolves. The shield eventually failed when the sheer power unleashed by the Loyalists' assault upon the kine-shield proved to be more than the individual Raptora Astartes could mentally bear. They fell exhausted to the terrible mental strain, one by one, until the psionic shield collapsed.