In 40K, there are many different factions, and while the Space Marines are the poster boys for the setting, there are other options.

Warhammer: The Horus Heresy is set 10,000 years before Warhammer 40,000, so in essence, it's Warhammer 30,000. Each son, now brought up in different environments had entirely different ways of thinking and waging war and they all set about reuniting the galaxy together until the Emperor decided to name Horus, the first found son as Warmaster, to lead the campaign in his stead while he returned to earth to continue other ventures for the betterment of humankind. The Emperor found his sons one by one as he set about reuniting the galaxy and the human race that had spread to the stars. These sons were mysteriously spread around the galaxy as infants, initially lost to the Emperor and brought up on different worlds, far from their intended guiding hand. Set 10,000 years before, the Emperor of mankind had united humankind's homeworld through force under his banner and had created twenty sons, the Primarchs, to lead legions of superhuman soldiers, the Space Marines. The Horus Heresy was originally a piece of lore and art detailed in the pages of the early editions of Warhammer 40,000.

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