The Diomedes was selected as the flagship of Battlegroup Kilo shortly after it was formed. Admiral Pikes, who was another veteran from the Human-Covenant War, had been absent from the confines of a UNSC ship for almost a year now and was happy to accept any that he may serve on. It was also here on the Diomedes that Spartan B-188 Devon acted as the commander of the Battlegroup's ground forces, and it was also the place where he died from brain cancer in 2568 CE. During the Battle of Arkasis, the Diomedes lead Battlegroup Kilo to do battle with a Brute fleet invading an Elite military outpost on the planet.