The Battle of Pluto was one of the earliest engagements of the Solar Wars, when the Traitor Legions, under Warmaster Horus, began encroaching upon the Sol System and the Imperium of Man's Throneworld of Terra during the closing years of the Horus Heresy in 010.M31. This campaign began as infiltration forces and probing raids from without, and Dark Mechanicum attacks and attempted break-outs from the besieged Traitor forces on Mars from within, to test the defences of the Throneworld of humanity. The Alpha Legion carried out multiple diversionary attacks of sabotage and preparation, which plunged the Sol System into complete chaos. Their true target, however, was the fortress moon of Hydra which orbited Pluto. Here, the Primarchs Alpharius Omegon would be confronted by Rogal Dorn, the Praet