The Mark of Chaos is a physical or spiritual sign of favour bestowed by one of the Dark Gods upon a devoted servant. It may appear as a brand seared into the skin, a rune that manifests unbidden upon armour or flesh, or a subtler alteration of body and soul, but always it marks the bearer as belonging to a particular power.
Each of the Chaos Gods grants its own distinct blessings along with its mark. The favoured of Khorne are filled with unquenchable battle-fury; those of Nurgle are gifted with grotesque resilience and freedom from pain; the chosen of Tzeentch are touched by sorcery and change; and the marked of Slaanesh are heightened to unbearable speed and sensation. To bear a mark is to be drawn deeper into a god's service, and its gaze.
Such favour is never without cost. Every blessing binds the recipient more tightly to a hungry master, and the gifts of Chaos have a way of spiralling beyond control. A warrior who accumulates too many boons risks losing themselves entirely, dissolving into a mindless Chaos Spawn. For the warriors of the Chaos Space Marines, a mark is thus both a prize and a peril, a sign of power that whispers always of the price still to be paid.