Space Marines (Warhammer 40,000)

In the table-top wargame Warhammer 40,000, the Adeptus Astartes (also known as the Space Marines) are genetically modified superhuman soldiers, the elite warriors of the Imperium of Man. Space Marines have been one of the starter armies in every box edition of Warhammer 40,000, Space Hulk, and Epic. They also feature heavily in other Games Workshop products, such as books, films, and video games, and are central to the universe's setting.

The background shown in Codex: Space Marines (Haines and McNeill, 2004) states that the Space Marines were the successors of the Thunder Warriors, the powerful but unstable superhuman warriors of the Emperor of Mankind used to reunify Terra after the Age of Strife. Following the galaxy-wide civil war known as the Horus Heresy, in which nine of the twenty original legions rebelled and betrayed the Emperor, the remaining Space Marine Legions were split into multiple "chapters", which are then each divided into ten "companies", traditionally with the 1st Company being elite veterans and the 10th Company beings composed solely of Scout Marines.

Space Marines are warrior-monks. From the time of their initiation to their deaths in battle, they spend their entire multi-century lives fighting in the name of the immortal God-Emperor of Mankind. They have been genetically and physically enhanced with gene-material and organ implants that ultimately derive from the Emperor's own flesh. They wear power armor and wield the finest weaponry available to the Imperium.

In the distant future, the Emperor of Mankind creates the twenty Primarchs: genetically engineered superhumans possessing immense physical and psychic power. Created from modified stocks of his own DNA, each Primarch was, in essence, one of his sons and each individual's genome serves as a template for a Legion of Space Marines. However, they were separated from him and their legions by the Chaos Gods while they were still in incubation and scattered on worlds spread through the galaxy. This would have a lasting impact on each Primarch and their Legion. During the late 30th millennium AD, the Emperor uses the Space Marine Legions to conquer the human-inhabited worlds of the galaxy, uniting them in the Imperium of Man and reuniting the Primarchs with their Legions. As the campaign draws to a close, a number of Primarchs under the leadership of the Imperial Warmaster Horus, and their Legions convert to the worship of the evil Chaos Gods and rebel against the Emperor during the Horus Heresy. The rebels are ultimately defeated and banished, though they continue to harass the Imperium as Chaos Space Marines. The Legions of Space Marines who remained loyal to the Emperor are restructured into smaller units called "Chapters" to make a future mass rebellion unlikely.

...
Wikipedia