For the foundational myth of the Warhammer 40,000 universe, the Horus Heresy had an extremely prosaic origin. The reason it exists, this massive fallen-paradise fairytale taking place 10,000 years in the setting's past, is that Games Workshop couldn't afford to put miniatures representing two different factions in a box back in 1988.
Rewind to the '80s. Adeptus Titanicus is Warhammer 40,000 with mechs, presumably developed in an attempt to come at BattleTech's crown. But a boxed set with 12 miniature Titans is expensive to produce, and to cut costs the two opposing forces of six giant robots have to be identical. A civil war is written into the setting's history to explain why the Imperium was fighting a mirror match against palette-swapped versions of its own mechs, and that civil war is called the Horus Heresy.
