Chapters were referred to as Fellowships within the Thousand Sons and the most of these the rebuilt legion was ever able to field numbered 10. However, at the conclusion of one particularly bloody campaign, the Thousand Sons had lost almost 900 Astartes, effectively wiping out an entire Fellowship. Rather than rebuild it, Magnus decided to reorganise and maintain the legion as one of 9 Fellowships, an organisation he referred to as the Pesedjet. Two special units were created within the legion; the elite Scarab Occult Terminators and the Hidden Ones of the Scout Auxillia. The legion’s command structure was adapted to include Proserine philosphy in the form of the Rehati (or the coven of Magnus) a second command-track in which the legion’s senior officers were ranked according to psychic ability and influence

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.

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.