Adversaries

The Imperium has many enemies, all of them reaching to crush humanity's throat beneath their filthy fingers. Whether they be the enemy within, the enemy without or the enemy from beyond, these beings are destroyed without mercy by the Imperium's armies.

Chaos
The servants of the Dark Gods are insanely faithful to the denizens of the warp, and are willing to do anything to appease their masters.

Cultists: these heretics are the most common of Chaos servants, scattered on almost every world, across the galaxy. Mostly, they are no threat, with light armaments keeping them relying on numbers, and they usually stay close to the ground, out of sight and out of mind. However, arbitrators and Guardsmen commonly go on raids that find evidence of activity in heretic cells, commonly beyond expectations. They are also the first force mobilised when the Chaos gods want to begin conquest or destruction of a planet.

Well-Known Foes: NA

Encountered On: Grahl, Midgard, Prometheus

Militia: whether they be sworn footsoldiers of Chaos or Imperial traitors, Chaos militia are far better trained and armed than heretics, sporting anything from outdated autoguns to paramilitary equipment like energy weapons. Militia are not to be underestimated, for they have the training to make up for their lower numbers, and they are just as capable as many veteran troopers in the Imperial Guard.

Well-Known Foes: Midgard PDF, Blood Pact, Sons of Khorne

Encountered On: Midgard, Prometheus

Traitor Marines: the toughest of Chaos' servants, these terrifying foes are also the least in number, as they have no way to make new Marines other than to convince Imperials to come to their side. They make up for their low numbers with centuries or even millenia of experiance behind their superhuman augments, and their unholy armaments are known to possess daemonic spirits, making the weapons themselves enemies to be feared.

Well-Known Foes: Many different Legions

Encountered On: Prometheus

Xenos
Also known as the enemy without, the alien comes in many different forms. While the Imperium exterminates dozens of alien species every year, there are many more, and several survive to continue to harass humanity. While Imperial propoganda paints aliens as inferior to humanity in every way, the truth is that this misinformation is to hide the simple fact: if not dealt with quickly and harshly, the Imperium would be overrun and destroyed by any of the numerous aliens arrayed against them.

Eldar: these enigmatic, humanoid creatures are, at first, physically similar to humans. Slender, not as heavily muscled and certainly more frail, the average eldar warrior is typically weaker than a human. They make up for this, however, with immense skill and stupendous agility, backed by their near-mythical technology. Eldar are the most likely to cooperate with humanity, as they see humans (mon'keigh in their tongue) as bumbling and young, but still with potential and hope to do the right thing. However, such alliances are often brief, short-lived affairs frought with tension that result in an immediate conflict after the greater threat is extinguished. To the eldar's distaste, not all of these fights are their victories.

Well-Known Foes: Farseer Iliyanda, Ranger Tahlza Ruama, Howling Banshee Exarch Myavel Kaspavin

Encountered On: Merov, Phalanx

Orks: the most barbaric of xenos the Imperium has seen, orks are known as the 'old foe', the enemy humanity has been battling practically since recorded time. Orks are physically intimidating, with iron-hard chords of muscle and tall, blocky frames. Their technology is astounding, as it is extremely crude, yet still functions with brutal effeciency. This, combined with their innate bloodlust and ferocity make an ork charge a terrifying sight in large numbers. The only relief that the Imperium has against the green tide is that the orks' desire for battle leads them to fight each other as often as they do other enemies.

Well-Known Foes: Warboss Hammafist

Encountered On: Phalanx

Radicals
The trickiest of the Imperium's enemies, renegades have renounced what has been set down as unbreakable Imperial law, methods and beliefs, branding them traitors and heretics. Yet despite these atrocities, they still swear that they serve the Imperium's best interests, despite their sinful methods, such as summoning daemons, committing tech-heresy, consorting with aliens and many other dark things.
 * The Black Regiment: though actually known as the 666th Covert Stormtrooper Regiment, these black ops specialists gained their name due to their stealthy nature. Known to outclass even Inquisitorial stormtroopers, these men are the most elite of the elite, known to take down Space Marines with few casualties.  The Black Regiment has since gone rogue, serving only the rogue Inquisitor Felroth Gelt, whom they are fiercely loyal to.  The Black Regiment, at last count, numbered at five-thousand soldiers, all of them veteran elites and extremely well armed.