The long-standing nemesis of the Space Marines, the forces of Chaos continually try to bring their foul gods to the real universe. Some of the staunchest anti-demon forces, the Grey Knights are always doing their best to stay ahead of them. Well, both sides of the conflict are getting new codices as well as releases. You can pick them up over in Games Workshop’s webshop.
From the website:
Smite the Daemon!
Chanting rites of banishment, the elite brotherhoods of the Grey Knights stride fearlessly into battle against the worst horrors of the galaxy. They wear blessed silver battle-plate and wield potent weapons of purgation, each a master-wrought relic whose worth can be measured in worlds. Every battle-brother of this Chapter is a powerful psyker, his will an adamant fortress and his mental powers a honed blade. As the galaxy becomes ever darker, the selflessly heroic Grey Knights fight unbowed.
Death to the False Emperor!
The Chaos Space Marines are superhuman traitors grown strong on the power of the Dark Gods. The ultimate nemesis of the Imperium, they surge from hidden lairs and hellish warp storms to fall without warning upon their victims. Murderous warriors clad in baroque power armour fight alongside teeming human Cultists and hulking Chaos Terminators; gargoyle-mouthed daemon engines stomp to war alongside growling battle tanks and hosts of malevolent entities spawned from the warp. Leading this malefic throng are the Champions of Chaos, evil warlords who have plagued the empire of Man for ten thousand years and more. Now, as the skies bleed raw Chaos energy, their time has come. Let the galaxy burn!