The Imperial Aquila is the double-headed eagle that serves as the supreme symbol of the Imperium of Man, emblazoned upon banners, armour, starships, and the very architecture of a million worlds. Its two heads gaze in opposite directions, and countless meanings have been ascribed to them across the ages: the union of Terra and Mars, the watch kept upon past and future, or the Emperor's dominion over both the material realm and the immaterium.
The symbol is said to date from the Unification of Terra, when the Emperor forged the scattered techno-barbarian tribes into a single dominion. Since then it has become inseparable from Imperial authority, a mark of loyalty and belonging worn with pride by servants of the Throne from the lowliest guardsman to the exalted warriors of the Adeptus Custodes.
To bear the Aquila is to declare oneself a subject of the Emperor and an enemy of all that would threaten Him. To deface or defile it is among the gravest of heresies, an act of blasphemy that invites swift and merciless retribution.