The Adeptus Custodes, known as the Legio Custodes during the era of the Great Crusade and Horus Heresy, and colloquially as the "Ten Thousand," is the Imperial adepta responsible for protecting the Imperial Palace and the physical body of the Emperor of Mankind, as well as serving as His most important emissaries, His companions, and the keepers of His many secrets. The Custodes is an elite cadre of genetically-engineered transhuman male and female warriors who are even more potent in combat than the Adeptus Astartes. They are to the Space Marines as the Emperor is to His primarchs, and it is rumoured that each was created by the Master of Mankind personally. His might permeates them, burns in their eyes and flows through their veins as surely as their blood. As such, the Adeptus Custodes are widely regarded as the deadliest warriors in the galaxy, Human or otherwise.

Where Space Marines represent the mass-produced, genetically-engineered soldiers of the Imperium of Man, the Adeptus Custodes are a force of individual warriors, each a bastion in their own right and a sentinel of unmatched capability and singular purpose created to counter any possible threat -- Human, alien or Daemonic. These warriors have stood in the presence of the immortal Emperor of Mankind since before the time of the Unification Wars. For ten thousand Terran years and more, the Custodians have stood watch over their lord and master, serving as the Emperor's personal heralds and praetorian bodyguard.