Warhammer 40,000 (WH40K) was originally a spin-off sci-fi version of the table-top game Warhammer Fantasy Battle, with its First Edition published in 1987; since the Second Edition in 1993, it has become a game in its own right. It is set in the 41st millennium, hence the number 40,000 and revolves around a dystopian, theocratic, totalitarian but stagnant galatic human empire known as the Imperium of Man and its conflicts with various other alien races in the galaxy as well as its own inner struggles. 

A typical scene of how an Imperial city looks like on Earth, known in the 41st millennium as Holy Terra.

Like most other sci-fi settings, the Warhammer 40,000's main protagonists are the forces of the Imperium, representing the "good", and the alien races as well as the forces of Chaos representing the "bad". Chaos is a spiritual force (much like a religion) in WH40K universe, with the four Chaos Gods representing different aspects of human sins, thoughts and emotions. Just like how Hell is portrayed in many fantasy settings, the Realm of Chaos (also called the Warp) is represented in WH40K as an alternate dimension only accessible via a black hole at the edge of the Milky Way Galaxy, known as the Eye of Terror. The Warp is completely separated from realspace, and in it, the laws of physics and time are null. It is also where the four Chaos Gods and their minions reside and they are the main antagonists of WH40K. Throughout the existence of the Imperium, many of its citizens and leaders have fallen to Chaos, and the term "heresy" is used in the 41st millennium to indicate everything from full Chaos corruption of individuals to simply speaking anything good about Chaos and caught by the authorities; and the punishment for heresy in the Imperium is most often by death.

Travelling into the Realm of Chaos (The Warp)

The four Chaos Gods in WH40K are:

- Khorne: Chaos God of Blood, War and Murder, Martial Pride and Honor. Humans follow Khorne or feed Khorne's sentience in the Warp through emotions such as hate, anger, war, rage, or killing. A Khorne worshipper is both an honorable combatant and a crazed bloodthirsty warrior. In the Realm of Chaos, Khorne is the mightiest and strongest God with the most followers and armies at disposal.

- Tzeentch: Chaos God of Change, Intrigue, Evolution, Sorcery, Hope, Evolution and Fate. Followers of Tzeentch are usually individuals of high intellect, charisma and a thirst for more knowledge, power, change and control. Called "The Architect of Fate", Tzeentch manipulates everything from his followers to even other Chaos Gods to fulfill his plans, and his appearance is constantly changing. 

- Nurgle: Chaos God of Decay, Disease, Destruction and Rebirth. Nurgle is the oldest of the Chaos Gods, and in general any humans suffering from illness or despair feed his sentience and can easily become his followers. Nurgle's ideology is simple: decay and rebirth are simply a cycle of life, and a Nurgle worshipper usually appears disgusting at best, but are immune to disease and physical weakness. 

- Slaanesh: Chaos God of Pleasure, Lust, Excess, Perfection, Passion, Pride and Self-Indulgence. Unlike other Chaos Gods, Slaanesh has no fixed gender and can choose to appear as both male and female. Slaanesh's followers are humans that desire more pleasure and perfection in all things, ranging from sex (Lust) to personal achievements (Self-Indulgence) but in the most excessive ways possible. 

The Chaos Gods. Clockwise from top left: Khorne, Tzeentch, Slaanesh and Nurgle

The people in the WH40K universe live in a time where daemons are physically real, human mutants are rampant and their Imperium is in eternal conflict to protect itself from the predators in the grimdark galaxy. Human life in WH40K is extremely cheap and expendable except for people holding important positions within the Imperium, but should the Imperium cease to exist, so will the human race. 

In addition to Chaos, in the 41st millennium the Imperium has to wage constant war to defend itself from various alien races, namely:

- Orks: Green-skinned, crude but aggressive aliens who reproduce asexually and possess technology almost on par with or comparable with the Imperium's. They only exist to wage war through massive conquests throughout the galaxy known as WAAAGHs, And, as it has often been said by Imperial scholars, it is war that keeps the Ork race strong, albeit fractuous. 

An Ork horde

- Eldar: A dying race. They used to be the largest galatic empire in the Milky Way until their doom in the 30th millennium by the birth of the Chaos God Slaanesh, who now owns almost every last Eldar's soul in the galaxy. Possessing technology far more advanced than the Imperium's, they seek to protect themselves from extinction through conquest and expansion, and this has caused them to be at war with the Imperium for millennia. 

A band of Eldar warriors

- Necrons: An ancient but extremely powerful race. The Necrons are humanoid robots have been asleep for more than 60 million years in countless Tomb Worlds hidden throughout the Milky Way, and they used to be the mightiest empire with unmatched technology in the galaxy before they went into hibernation. Now gradually awakening, they seek to enslave the galaxy once more and restore the ancient Necron Empire into its old glory.

A Necron Dynasty

- Tau: A nascent race of aliens who is rapidly expanding in terms of empire size and technological wonders. The Tau follows a religion known to them the Greater Good (much like Communism), and seeks to persuade or coerce other races (including humans) to follow it too. They are usually peaceful and negotiative when it comes to conflict with other races, but when diplomacy fails they will then unleash war upon their foes and conquer or absorb them into the Tau Empire.

Tau Fire Warriors and a Battlesuit

- Tyranids: Even more aggressive and numerous than the Orks, Tyranids are monstrous aliens whose sole purpose is to absorb all biomatter in the galaxy into their own genetic material for evolutionary advancement. A planet suffering from a Tyranid invasion often finds itself lifeless and drained of all resources in the aftermath, a byproduct of the race's consumption of all planetary lifeforms from flora and fauna - including humans. 

A Tyranid Lictor

Yet, despite all the above-mentioned threats to its existence, the Imperium prevails through countless heroic deeds and sacrifices over the millennia. The backstory for the WH40K setting will be provided in Part II. 

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