The Adeptus Custodes, known as the Legio Custodes until after the Horus Heresy, is the Imperial Adepta responsible for guarding the Imperial Palace and the physical body of the Emperor of Mankind. They are a group of 10,000 genetically-engineered superhumans who are even more potent in combat than the Adeptus Astartes and who serve as the Emperor's personal guardians and praetorian bodyguard. Since the Emperor's internment in the Golden Throne following the Horus Heresy ten millennia ago, they have taken a far more limited role in the Imperium of Man. They almost never leave Terra and only very rarely leave the Imperial Palace. Only the Adeptus Custodes decide who may enter the Sanctum Imperialis of the Emperor at the heart of the Inner Palace, and when they may do so.
The Custodes were founded by the Emperor of Mankind at an unknown date during the Unification Wars on Terra in the 30th Millennium following the Age of Strife. During that time they had yet to be given the title of Adeptus Custodes and were instead known as the Emperor's Companions and later on as the Legio Custodes.
They are the "Dread Guardians," and the only living beings permitted to walk beside the Emperor. The Custodes were the greatest group of physiologically and psychologically-enhanced troops the Emperor had ever created, ten thousand strong and the best trained, most disciplined and most vigilant watchmen in the Imperium. Now ensconced deep within the Sanctum Imperialis on Terra, they watch over the corporeal body of the Emperor.
Before the Horus Heresy, a unit of Custodes known as the Emperor's Companions used to accompany the Emperor at all times, even when he retired to his private chambers. During the Great Crusade, these great warriors always kept a contingent with the Emperor for his protection, but also travelled individually as his personal envoys. Even in the days before the Heresy, it was a rare and blessed thing to see a member of the Legio Custodes, and due to the confinement of the Emperor to the Golden Throne, it has become an almost unthinkable event. Even during the Battle of Terra, a group of Custodes joined the Emperor when he made his assault upon Horus' flagship, the Battle Barge Vengeful Spirit.
Since the death of Horus and the mortal wounds he inflicted upon the Emperor, the Legio Custodes, renamed the Adeptus Custodes after the Reformation of the Imperium conducted by the Ultramarines Primarch Roboute Guilliman in the 31st Millennium, have taken a more limited role by acting solely as the protectors of the physical form of the Emperor on Terra and the primary providers of security for the Imperial Palace.
The Custodes are slightly larger and stronger than even the superhuman Space Marines. The uniforms of the Custodians are traditional but effective: boots, leather breeches and long black cloaks. Their torsos are naked and corded with tattooed muscle. Their tall, brazen helmets, ominous and all-enclosing, give them a threatening and impersonal appearance; ancient and ornate, the helmets incorporate advanced protective equipment and communicators. Originally the traditional colour of the Custodes was red; following the confinement of the Emperor after the defeat of Horus, it was changed to black, the colour of mourning. The weapons carried by the Custodes resemble spears, but in fact they are laser weapons built into the shape of spears. The spears are built to resemble the original guardian spears, carried by the Adeptus Custodes before the Emperor's confinement. The guardian spear is closely associated with the Custodes, and is depicted on their banners and badges.
Although the Custodes were among the first genetically-modified warriors to be created by the Emperor during the Unification Wars of the late 30th Millennium, they were never intended to be part of a conquering army; such a role was to be filled by the later Legions of the Astartes. This is revealed both in their mindset and training: while Custodians share a genetic kinship with one another, they do not foster the same spirit of almost religious brotherhood that is instilled within the Astartes in order to function together as a unit. Indeed, each Custodian prepares and inspects his wargear individually, rather than as part of a unit ritual. The individuality of each Custodes is further promoted by the fact that the genetic engineering processes required to produce them is not as refined or as simple as that of the Astartes and thus they are not "mass-produced" as the Astartes are; meaning that each Custodian is a unique investment for the Imperium who undergoes a genetic enhancement process that has been specially adapted to his particular physiology and genome. If the Space Marines are the result of an "assembly line" process of genetic modification and enhancement, the Custodians are custom models.