In the myriad planets of the galaxy, beyond the million worlds held by the Emperor's light, there exist alien creatures whose origin is not of man. These abberations are created outside the cradle that is Holy Terra, and as such, are deviant innately sinful existences. They envy the holiness of the human form and the purity of human spirit, as well as the prosperity and might of the Imperium of man, so they seek to desacrate it, to drag our divine Empire int the same pit from which they crawled from.
Their forms are many, from the brutish Orks, the deceitful Eldar, the weak T'au, and the vile Tyranids, yet they are all enemies of the Imperium, seeking to destroy it, albeit pitifully.
The brutish Orks. The greenskin menace are a barbaric bunch, always waging war and trying to drag the Imperium into it. Do not underestimate them just because of their simplicity, for they are the Imperium's oldest enemy, with the Emperor and his Primarchs themselves having fought off many waaaghs of Orks in their conquest of the galaxy during the Great Crusade. They even nearly toppled the Imperium during the war of the Beast, ending the greatest age of the Imperium since the Horus Heresy
The most elusive of the xenos. The eldar are an ancient race that preys upon innocent men and women of the Imperium for their nefarious schemes. They gladly sacrifice cities and even entire planets of humans for the barest few of their own kind, completely disregarding the pricelessness of every human soul. Worse yet are the raiders, who capture humans and taken to their dark cities to be eternally tormented to atisfy their depraved desires.
The disgusting Tyranids. These swarms of creatures are not easily described, as they come in the widet variety of forms. nevertheless, their behaviors stay the same. They target the prosperous worlds of the imperium and seek to consume all upon their surface, from the smallest of plants, to the massive hive-cities housing trillions of human souls. Their ravenous hunger knows no end and no bounds, consuming all in their way.
The T'au are a pathetic upstart race desperately trying to carve aplace in this mad galaxy. A new player in the great game of the milky way. They barely understand the greater occurences of the galaxy, and continously underestimate the dangers therein. Still they persist, and they dare to usurp the Imperium as the dominant Empire of the galaxy.