The Imperialis symbol, a winged skull, dates back to the Horus Heresy when it was used by the Loyalist Space Marine Legions and the Imperial Army troopers in recognition of bravery and loss. Later, it became a badge of honour borne by Loyalist units during the Heresy. Now, in the latter 41st Millennium, it is used more to symbolise the righteous victory of the Imperium of Man over its many enemies in general and is awarded to Space Marines as a badge of honour following a victorious battle. Rather than being presented as a medal it is often carved into the chest armour plate of a Space Marine's Power Armour, replacing the standard Aquila symbol, although it also appears on Space Marine weapons, banners and finials. The Imperialis, especially when incorporated into a suit of Power Armour's chest plate, is often made from precious metal or some other rare substance inlaid into the surface of the armour so that it stands out to a greater degree.
The wings of the skull represent the wings of the Aquila, the double-headed eagle that is the official symbol of the Imperium, while the skull is a common motif in Imperial iconography which represents the sacrifice of the Emperor of Mankind for humanity's salvation. An ancient symbol with its origins in the Horus Heresy, the Imperialis was first used by the Loyalist Space Marine Legions as an army badge, denoting their continued devotion to the Emperor in the wake of their brethren's treachery. Since then, it has come to stand as a symbol of the loyalty a Space Marine has toward his Chapter and the Emperor, and is usually only worn by those that have sworn and completed numerous oaths of devotion to the Emperor to prove themselves. These Space Marines are favoured by their peers in matters of importance and trust. Chapter Masters also usually only recognise a Battle-Brother’s right to bear the Imperialis if they have overcome some form of temptation and had their loyalty tested, surviving the experience with their faith in the Emperor, the Imperium and their Chapter unbroken.
As a result of the corruptive influence of Chaos or xenos to which even the Adeptus Astartes are not immune, Space Marines who have been awarded this honour are often selected by their Chapter to hunt down such foes, such as powerful Chaos Sorcerers or alien psykers. Even when the chance of subversion is negligible, many Space Marine commanders choose such Battle-Brothers for special missions, as they have complete faith that those who earned the Imperialis will always carry out their duty. Some of the more sinister of the Imperium's foes also recognise what bearing the symbol of the Imperialis means for a Space Marine, and it can mark a Battle-Brother for special treatment by xenos or the Forces of Chaos.
- Deathwatch: Rites of Battle (RPG), pp. 206-207
- The Horus Heresy - Book Four: Conquest, pg. 95