A Two-Headed Eagle that symbolizes the Pax Imperialis, the oath that binds together all Imperial servants under the banner of the Emperor Of Mankind in Warhammer 40k.
It is a universal symbol within the Imperium Of Man and more than anything else represents the dominion of the Emperor. The double-headed eagle also represents the union of the Adeptus Mechanicus of Mars and the Emperor or between Mars and Terra.