Diocletian Coros

Diocletian Coros was a Prefect of the Hykanatoi and served as a part 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 during the 30th and early 31st Millennia. He fought in the Imperial Webway War under the command of Tribune Ra Endymion. Diocletian's ultimate fate is unknown.

History
Following the Crimson King's folly, when the Thousand Sons Primarch Magnus the Red attempted to warn the Emperor through sorcerous means of Horus' corruption and eventual perfidy, the wards placed on the Imperial Webway deep within the Imperial Palace's dungeons were breached, which allowed the legions of Chaos to encroach upon the Emperor's Palace. Five years into the war, things began to go badly for the Imperial forces, as the neverending legions of Chaos threatened to breach the sanctity of the Palace's dungeons. Diocletian was charged with organising a fresh wave of Imperial forces to resist the encroaching Forces of Chaos, which included the condemned prisoners of the fallen Imperial Knight House Vyridion, who would aid the beleaguered Imperial forces at Calastar (the derelict Eldar city, nicknamed the 'Impossible City') just as the Chaos hordes began to overrun it.

As things looked the most dire for the defending Imperial forces, the Emperor Himself was finally able to leave the confines of the Golden Throne, following the sacrifice of one-thousand psykers, in order to cover the retreat of the decimated Imperial forces and to confront the foul Daemonic entity known as Drach'nyen. During this final epic battle, Diocletian fought at the sides of both Ra Endymion and the Emperor Himself, and eventually fought his way back to Terra and escaped the confines of the Imperial Webway just before the Emperor sealed the portal, effectively ending the war within the webway. Following the battle, the Emperor began to show Diocletian visions of both His and Terra's past, as much as he had done with Ra. This could be an indication that Diocletian was raised to the status of Tribune following Ra's disappearance into the webway. The ultimate fate of Diocletian is unknown in official Imperial history.

Post-Heresy
Following the Horus Heresy, there were accounts, written in arcane script and buried in the deepest vaults of the Inner Palace, that tell of detailed testimony from the oldest of the Adeptus Custodes' order, now all long dead. Some of the greatest of them all, such as - Diocletian Exemplar, Thanassar, even Constantin Valdor himself - were said to have had dreams in which knowledge was given. To a Custodian, dreams do not mean the same to them as they do others. During their ordinary lives, Custodians do no dream at all, for something in their psyche changes after their transformation, and whatever purpose dreaming has for the mortal psyche is made redundant by the alternation. But there were exceptions, legendary ones. They were spoken of carefully, reverently, for it was in the form of dreams, long ago, that the Emperor's will was made most clearly manifest to his Custodians.

Diocletian would later go on to pen his seminal work, The Master of Mankind, a tome which would still be read by the Adeptus Custodes some ten millennia later.