A psyker is an individual whose mind can channel the raw energy of the Warp, the parallel dimension whose tides give rise to all sorcery and psychic phenomena in the setting. Rather than manipulating matter directly, a psyker acts as a living conduit, drawing immaterial force into reality and shaping it into effects such as telekinesis, foresight, pyromancy, or the projection of thought across vast distances.
The gift is rare, inconstant, and profoundly dangerous. Because the Warp is also the domain of hostile entities, an untrained or overreaching psyker risks catastrophe: uncontrolled surges, physical corruption, or outright possession. For this reason, human society treats latent talents with suspicion, subjecting them to rigorous testing, sanctioned training, or execution depending on the culture involved.
Among humanity, sanctioned psykers serve as battlefield officers, astral navigators, and long-range communicators, their abilities considered indispensable despite the peril they carry. Comparable talents appear across many species, each shaped by its own biology and philosophy, from disciplined xenos mystics to feral witch-cults. Power varies enormously, measured by informal grades that range from barely flickering sensitives to rare individuals capable of reshaping the course of a battle. In every case, mastery demands unbending willpower, since the true adversary is often the psyker's own mind laid bare to the immaterium.