Ra Endymion was a Tribune of the ancient Legio Custodes, the elite bodyguard of the Emperor of Mankind, and close confidant of the Master of Mankind during the Great Crusade and Horus Heresy. As a Tribune, Ra Endymion commanded the force of Custodians who fought in the War Within the Webway against the servants of Chaos during the Heresy, following the folly of the Thousand Sons Primarch Magnus the Red.
The Crimson King's use of forbidden sorcery to communicate with his father, the Emperor, in order to warn Him of Horus' fall to the Ruinous Powers, shattered the wards of the Imperial extension into the Webway deep beneath the Imperial Palace, which opened the way for the daemonic legions of the Ruinous Powers to attempt to force their way into reality through the portal beneath the Imperial Palace.
Ra is one of the only living souls who learned the entirety of his master's dreams and ambitions for humanity, a great gift provided as recompense for his ultimate fate. During the War Within the Webway, Ra Endymion saved Mankind by becoming the vessel of flesh that caged the daemon Drach'nyen. After being impaled by the daemonic sword at the command of the Emperor, the Master of Mankind ordered his most faithful praetorian to run forever into the Webway, as far as he could, imprisoning the daemon for as long as he lived. Endymion's fate after this event is unknown.
Ra Endymion was born on Terra during the Unification Wars to Koja Zu, the Minister of the Anuatan Steppes, around the time of the Battle of Maulland Sen. Ra's mother was executed by Captain-General Constantin Valdor for the high crime of stealing Terra's last remaining ocean when Ra was only four standard years old. He was taken after her death by the Legio Custodes as a potential candidate to join their ranks.
The young boy received the name "Ra" in honour of the ancient Gyptian sun god and eventually rose to become the leader of Squad Dynastes, later dubbed the Lords of Terra by the Emperor Himself. This squad was composed of twenty sons and nephews who had been taken as tributes from various defeated Terran rulers and warlords. By the opening years of the galaxy-wide conflict known as the Horus Heresy, Ra had earned eight-hundred and seventy-one personal names in honour of valorous deeds and was raised to the rank of Tribune.
Following the Crimson King's folly, when Magnus the Red attempted to warn the Emperor through sorcerous means of Horus' corruption and eventual perfidy, the wards placed on the Imperial extension of the Webway deep within the Imperial Palace's dungeons were breached, which allowed the daemonic legions of Chaos to encroach upon the Emperor's Palace. Five Terran years into the war, things began to go badly for the Imperial forces, as the never-ending legions of Chaos threatened to breach the sanctity of the Palace's dungeons.
With the deaths of both Tribunes Kadai Vilaccan and Jasac, Ra Endymion was given sole command over the Custodians fighting the War Within the Webway. When the derelict Aeldari city within the Webway known as Calastar (nicknamed the "Impossible City") fell to the daemonic hordes' assault, the Emperor was finally able to leave the Golden Throne, following the sacrifice of one-thousand psykers, in order to cover the retreat of the crippled Imperial forces and to confront the daemon Drach'nyen.
In a final confrontation, the daemon took the form of a Stone Age war-chief, the first human to have taken another human's life, and the Neverborn impaled the Emperor, claiming that it would be His death. But the Emperor tore the daemon, now in the form of a great daemonic sword, from His body and with armour-boosted strength and powerful telekinetic force, hurled it from His grip at Ra. The sword impaled the Custodian, causing the daemon to be bound within him. The Tribune didn't demand any explanation or apology. Ra was born to serve, raised to obey and chosen for the greatest illumination preceding the darkest duty.
The sword quickly disappeared, for it was now bound within the Custodian's body, caged within Ra's flesh. The daemon within him could not be killed, not even by the Emperor Himself, only imprisoned. As the last remaining Imperial forces retreated from the Imperial Webway, the Emperor ordered Ra to run deep within the myriad tunnels of the Labyrinthine Dimension as far as possible, separating the daemon from his master, which meant another day that the Imperium would stand unbroken. Ra fled deep into the Webway at his master's command, never to be seen again, his ultimate fate unknown.
- Master of Mankind (Novel) by Aaron Dembski-Bowden