Possession is the seizing of a physical body by a daemon, one of the primary means by which the Neverborn breach the barrier between the Warp and reality. Denied bodies of their own in the material universe, daemons covet living hosts through which to walk, fight, and work their patrons' will.
A daemon may force its way into a victim, or it may be invited, drawn across the veil by ritual, sacrifice, and the spilling of blood. Psykers are especially vulnerable, for their minds already open onto the immaterium, and an untrained or overconfident witch can be overwhelmed and hollowed out in an instant. The possessed body is often warped by the entity within, twisting into something that blends mortal and daemonic forms.
For the servants of Chaos, possession can be a coveted gift rather than a violation. Certain Chaos Space Marines willingly open themselves to daemonic union, becoming Possessed warriors whose flesh flows and reshapes with the fury of the thing inside them. However it occurs, the result is dreadful: a foe that combines physical strength with unnatural power, and which cannot be truly slain, for destroying the host only banishes the entity back into the Warp to hunger anew.