Diocletian Coros was a Prefect of the 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 was a member of the Custodians' Hykanatoi caste.
He fought in the Imperial War Within the Webway under the command of Tribune Ra Endymion, and managed to escape back into realspace before the Emperor sealed the portal in the Imperial Palace's dungeons, though his ultimate fate remains unknown.
Coros was one of the few Custodians still capable of dreaming, which they believed represented a way for the Emperor to make His will known to His praetorians after His ascension to the Golden Throne.
Coros would pen a tome, The Master of Mankind, which would contain many of his dreams and prior interactions with the Emperor that would still be available to the Adeptus Custodes of the 41st Millennium.
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 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 hordes of daemons threatened to breach the sanctity of the Palace's dungeons and spill out onto the surface of the Throneworld itself.
Diocletian Coros 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 Aeldari city, nicknamed the "Impossible City") just as the Chaos hordes began to overrun it.
As events turned 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 battle, Diocletian fought at the sides of both Ra Endymion and the Emperor, 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, 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 Master of Mankind
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 -- Diocletian Coros, 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 as they do others. During their ordinary lives, Custodians do not 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 alteration.
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 Coros would pen his seminal work, The Master of Mankind, some time after the end of the Heresy, a copy of which could still be read by the Adeptus Custodes some ten millennia later. It detailed many of his interactions with the Emperor, including the visions and dreams he believed the Master of Mankind had given him.
- Master of Mankind (Novel) by Aaron Dembski-Bowden
- Watchers of the Throne: The Emperor's Legion (Novel) by Chris Wraight, pp. 49, 82