Apollo Diomedes is the 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. During the Second Aurelian Crusade, he 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 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 along with 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 have ever seen, Diomedes does have a fatal flaw: pride. Because of this, along with 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, it nearly brings him to ruin when he is finally faced with the truth. On the Space Hulk the Judgement of Carron, Diomedes begins to fall to despair when he realizes that he has been serving a traitor for many years. It is the intervention of the Ancient who has taken a vow of silence who prevents his fall to Chaos. The Ancient reveals himself to be Tarkus who says he had to slay is his friend Avitus who fell to Chaos, and that he now sees the same signs in Diomedes and will end him if he has too and stop the corrupt Chapter Master by himself if necessary. Tarkus reminds Diomedes that though they have served a traitor these many years, even he cannot take away their duty to the Emperor. Emboldened by his speech Diomedes leads his men off of the Space Hulk and defeats Kyras, saving the sector as well as the Chapter from destruction.
Apollo Diomedes still continues to serve with the Blood Ravens Chapter as the Captain of its Honour Guard, which now serves Gabriel Angelos.
In the series of Dawn of War PC Games, Diomedes is voiced by Keith Szarabajka.