The Kai Gun is a huge, two-handed, ballistic weapon of archaic design similar to a Bolter that was developed following the loss of the Forge World of Kai to Warp Storm Gae-sann in the 34th Millennium. Possessing an advanced industrial culture, the Machine Smiths of Kai bartered their skills for a measure of protection, learning to fashion weapons in the Warp that they could not have conceived before.
Their ploy eventually failed and the daemon hordes of the Warp descended upon them to fight for possession of these new domains. All that remained after the destruction of Kai was the weapons they had once built to appease their slayers. The Kai Gun is so large that a normal man would be unable to lift it, but one which a Chaos Lord or Daemon Prince would have no problems wielding. The gun is a psychic catalyst, turning the hate and malice of its wielder into tangible bolts of energy.
- Codex: Chaos Space Marines (3rd Edition, 2nd Codex), pg. 20