The Proximan Betrayal was a battle during the early Great Crusade in M30. The battle began during the ceremony that saw Proxima formally surrender to the fledgling Imperium. During the ceremony, anti-Imperial rebels attempted to assassinate the Emperor himself. The Master of Mankind himself was wounded by a Vortex weapon, and a desperate battle broke out that saw the III Legion of the Legiones Astartes and Custodian Guard defend their leader against insurrectionists. Thanks to the effort of his Astartes, the Emperor was able to break free of the rebel encirclement attempt and eventually crush the rebellion with the invocation of Exterminatus.
In the aftermath of the battle, the Emperor rewarded the III Legion with the honor of wearing his personal symbol on their armor in recognition of their loyalty and bravery.