From Lexicanum

The Battle of Charaxdis was a large naval engagement in which the Mars Class Battle Cruiser Pax Imperium was heavily damaged. Pax Imperium was the flagship of Lord Commander Solar Macharius, and it was this battle that finally forced him to move his command over to the much more heavily armed and armoured Emperor Class Battleship Lord of Light.¹

Sources

• ¹: Codex: Imperial Guard (3rd Edition, 1st Codex)