Titan

Titan is Saturn's largest moon and the hidden fortress-world of the Grey Knights. Malcador shifted the moon into the Warp during the Heresy; when it returned, a full Chapter had already been raised inside the Citadel of Titan. Cryovolcanoes vent ammonia into a thick orange sky, and the moon's batteries sit just behind Terra, Mars, and Cadia in the Imperium's defensive order.