Apollo Diomedes was the former Captain of the Blood Ravens Space Marine Chapter Honour Guard and the right-hand man of the Chapter's former Chapter Master and Chief Librarian, Azariah Kyras, before eventually becoming one of the Chapter's Chaplains in the wake of Kyras' corruption by Chaos and death during the Third Aurelian Crusade at the hands of Gabriel Angelos and his uncorrupted brethren.
Diomedes successfully commanded the Blood Ravens' Veteran 1st Company for almost two standard centuries. His most famous victory came on the ravaged Penal World of Obscurus where he and his brothers defeated the Ork Warboss Manstompa Megakilla and the Chaos Sorcerer Anupharis the Cruel. During the battle, Anupharis laid a curse upon a Combat Shotgun owned by the Chapter which would later cause the deaths of many civilians whenever the weapon was wielded in battle. After that victory, Diomedes was elevated to the command of the Chapter Master's own Honour Guard and became Azariah Kyras' most public supporter and right hand.
During the Second Aurelian Crusade, Diomedes arrived in Sub-sector Aurelia and ordered all Blood Ravens forces in the region to stand down in their operations against the Forces of Chaos, an order that the Aurelian Force Commander Aramus and Captain Gabriel Angelos ignored. As Diomedes grew increasingly sterner in his orders, Force Commander Aramus and Angelos continued to resist the forces of the Black Legion, with whom the Chapter had recently avoided open conflict for an unknown reason.
As his investigation continued, Aramus discovered to his horror that the source of the Chaotic corruption within the Chapter was none other than Azariah Kyras himself, who had fallen under the sway of the Greater Daemon of Nurgle named Ulkair on the ancient world of Aurelia over a millennium before and spread his taint to Galan aboard the Space Hulk Judgement of Carrion. Kyras had escaped the doomed planet Aurelia when it was consumed by a Warp Storm aboard the Space Hulk with the aid of Ulkair and had returned to the Chapter with his dark seed of corruption when he met with a Blood Ravens expedition exploring the Hulk that included Galan.
During open conflict with Captain Diomedes' forces in their base-camp on Calderis, Sergeant Aramus attacked Apothecary Galan, slaying the Traitor and his Chaos-tainted bodyguard of Terminators, a battle which ended in a confrontation with Captain Diomedes himself. Reluctant to accept the corruption of his political patron within the Chapter, Diomedes found himself torn but let Sergeant Aramus and his comrades leave Calderis and continue their operations against the Black Legion warband of Araghast the Pillager. Captain Diomedes, joined by Cyrus, Martellus and unknowingly Tarkus, led the uncorrupted portions of the Blood Ravens Chapter during the Cleansing of Aurelia that eventually lead to the death of Azariah Kyras and the appointment of Gabriel Angelos as the new Chapter Master of the Blood Ravens.
Named by Kyras as the greatest warrior the Blood Ravens had ever known, Diomedes did possess a fatal flaw: pride. Because of this, as well as his long service under Kyras, it took a long time for Diomedes to accept Kyras' betrayal of the Emperor and his Battle-Brothers, much to the annoyance of the irascible Veteran Scout Sergeant Cyrus as well as Techmarine Martellus. In fact, these obstacles nearly brought him to ruin when he was finally faced with the truth. On the Space Hulk Judgement of Carron, Diomedes began to fall to despair when he realised that he had been serving a Traitor for many standard years. It was the intervention of the Ancient who had taken a vow of silence that prevented his fall to Chaos. The Ancient revealed himself to be Tarkus who explained he had to slay his friend Avitus who fell to Chaos, and that he now saw the same signs in Diomedes and would kill him as well if he had to and stop the corrupt Chapter Master by himself if necessary. Tarkus reminded Diomedes that though they had served a Traitor these many years, even Kyras could not take away their duty to the Emperor. Emboldened by this speech, Diomedes led his men off of the Space Hulk and defeated Kyras, saving the sector as well as the Chapter from destruction.
Apollo Diomedes continued to serve for a time as the Captain of the Blood Ravens Honour Guard, which now protected the new Chapter Master, Gabriel Angelos.
During the conflict on the Imperial planet of Cyprus Ultima, and later the world of Acheron, it was revealed that Diomedes had relinquished his position within the Chapter Honour Guard and taken up the role of a Chaplain. When the Blood Ravens relieved the siege of Varlock Keep, Gabriel Angelos gave Diomedes orders to reinforce the battered garrison. Together with the Imperial Knights' House Varlock and its leader Lady Solaria, Chaplain Diomedes led a joint offensive against the besieging Ork forces, forcing them from Imperial territory and into the Cyprus Badlands. Diomedes was later redeployed to Starfort Helios where he and his forces tried to fend off Gorgutz's WAAAGH! from breaching the Imperial blockade, but the starfort was destroyed when the planet Acheron reemerged from the Warp and collided with Helios.
When Helios fell, Gabriel Angelos believed Diomedes and his brothers lost. In honour of his memory and to give him his last rites, Angelos ventured to the world of Acheron, where pieces of Starfort Helios had fused with the planet's crust. It was only after investigating the crash site that Angelos and the Blood Ravens discovered evidence indicating some of their brethren had survived the starfort's destruction. With a small team of Terminators, Angelos managed to locate Diomedes and save him from the Eldar. Together they proceeded to destroy a nearby Eldar staging ground, discovering that the Eldar had used the Webway to bypass Inquisitor Holt's blockade of Acheron in the process.
In the Dawn of War series of games, Diomedes is voiced by Keith Szarabajka.