Thousand Sons were the XV Legion of the Legiones Astartes, led by Magnus the Red – the cyclopean Primarch obsessed with knowledge and mysticism. They hailed from the planet Prospero, home of scholars, mystics, and powerful psykers. The Legion was filled with psykers, making it unique among the Imperium’s forces.

Their fate was sealed by the Edict of Nikaea, which forbade the use of sorcery. Magnus attempted to warn the Emperor of Horus’ betrayal, but his act led to the destruction of Prospero by the Wolves of Fenris. After this massacre, the Thousand Sons turned to Tzeentch, the god of change and sorcery.

In the Horus Heresy system, the Thousand Sons have access to the Cult Arcana – five schools of sorcery: Pavoni, Corvidae, Raptora, Athanaean, and Pyrae. Their units include Sekhmet Terminators, Khenetai Occult Blades, Osiron Dreadnoughts, and heroes such as Ahzek Ahriman, Magistus Amon, and Magnus himself.