The Imperialis is a Space Marine Honour Badge.
Typically it appears as either a winged skull or a pair of linked wings. Usually the Imperialis is worn on a Space Marine's chest plate, though in some cases it can appear on banners or boltguns, though this is not as common and does not send as bold a message about their loyalty to those around them. The Imperialis, especially when part of the 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.
An ancient symbol with origins in the Horus Heresy, the Imperialis was first used by the loyalist Space Marine Chapters as an army badge, denoting their continued devotion to the Emperor. Since then, it has come to stand as a symbol for the utter 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 unbroken.
Due to the influence of Chaos or xenos where the Adeptus Astartes are not immune, the Space Marines with this honour are often selected to hunt down such foes, such as powerful Chaos Sorcerers or xenos witches. Even when the chance of subversion is negligible, many Adeptus Astartes commanders choose such men, as they have complete faith that they will complete their duty. Some of the more sinister of the Imperium's foes also recognise the Imperialis, and it can mark a Battle-Brother for special treatment by xenos or Chaos.