Codex: Grey Knights is an expansion Codex. It was released in 2014, for the 7th Edition of Warhammer 40,000.
The Grey Knights were forged in the dark days of the Horus Heresy — their sacred duty to act as the Emperor's ultimate sanction against the armies of the Immaterium. Peerless psykers and fearsome warriors, the Grey Knights Space Marine Chapter are master Daemon hunters and often Humanity's final line of defence against the powers of Chaos. It is an unending war against an immortal foe, however the Grey Knights are an army like no other, and they fearlessly face their foes with ready blades and blazing storm bolters.