The Adeptus Custodes, known as the Legio Custodes during the era of the Great Crusade and Horus Heresy, 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 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 his 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.
During the Great Crusade these resplendent warriors guarded the Emperor wherever He went and swore to give their lives to protect His. After His incarceration in the Golden Throne following the Horus Heresy ten standard millennia ago, they have taken a far more limited role in the Imperium of Man. They almost never leave Terra and only very rarely are seen outside the Imperial Palace as they watch over their recumbent master and patrol the precincts of the Emperor’s palace, ever vigilant of threats. It is only the Adeptus Custodes who decide who may enter the Sanctum Imperialis of the Emperor at the heart of the Inner Palace, and when they may do so.