The Sensei background was introduced in the Realm of Chaos books and referred to later in the 3rd Edition Warhammer 40,000 Rulebook, which removed them from the canon.
The Sensei are the sons or descendants of the Emperor, and are the counterparts to the Champions of the Chaos gods: the Sensei are Champions of the nascent Star Child. The Sensei are hunted down by the Ordo Malleus of the Inquisition who consider the Sensei a threat to the order of the Imperium, and possibly in league with Chaos. The Sensei are also a vital part of the Illuminati's plan to save humanity, by rejuvenating the Emperor; the Illuminati gather the Sensei and protect them from the Inquisition, preparing them for "the final war with Chaos".
Between the two Realm of Chaos books, the exact nature of the Sensei is contradicted: in one version they are the direct sons of the Emperor, fathered over his vast lifetime, and are to be sacrificed to the Emperor by the Illuminati, in the same way many millions of psykers have already been sacrificed to feed the Emperor. All of the remaining Sensei are to be offered to the Emperor as a sacrifice at the moment his will finally breaks. The Emperor will be rejuvenated, and reborn as the Sensei-Emperor to again lead his race in person.
"The final war with Chaos" that the Sensei are preparing for is said to be a sham, invented by the Illuminati.
The Inquisition has discovered that captured or killed Sensei can be rendered down to manufacture the negative psychic energy used in psycannon bolts and Psyk-out weapons. The only other source of this energy is the byproducts of the Emperor's metabolism.
The second version effectively writes out the Illuminati and their plot, introduces the Star Child, and makes the Sensei the Champions of the Star Child:
As the Powers of the Warp have their own mortal champions, so too does the Star Child. The Star Child's champions are known as the Sensei. Although they do not necessarily know their true identity, they are actually descendants of the Emperor. Nor is the Emperor himself aware of his immortal descendants.
The Sensei have inherited great power from the Emperor. They are immortal; although they can be killed, they do not age, and possess amazing powers of recovery. They are also by their nature protected from the Chaos Powers. Because they harbor none of the emotions or concepts embodied by the Chaos Powers they are largely invisible to them, and can draw on the energies of the warp to use their psychic powers without risk of attracting daemons or other malicious forces of the warp. Due to their harmonious relationship with the warp, the Sensei are invisible to psychic senses, even to those of the Emperor.
Sensei wander throughout the galaxy. Their origins are unknown to the Inquisition, who view them as dangerous mutants and heretics. In contrast to the forces of the Imperium, Sensei are natural rebels. As well as being enemies of Chaos, Sensei are enemies of oppression. They and their followers operate as outlaw bands, and appear across the Imperium to fight against repression and injustice. In this way, they often come into conflict with the Imperium.
The Imperium regards the Sensei as dangerous bandits and nihilists who, if not actually in league with Chaos, are weakening the Imperium's defense against Chaos. Because of their powers and militantly anti-authority natures, they are hunted down and killed by Imperial forces, which in turn forces the Sensei to operate as outlaws.
Sources
- 1Inquisition War Trilogy (Novel Series), Ian Watson, Black Library Publishing
- 2Realm of Chaos: Slaves to Darkness and The Lost and the Damned
- 3Warhammer 40,000 3rd Edition Rulebook, p. 272