Board Thread:Warhammer 40k General Discussion/@comment-25149066-20140730050740/@comment-25177957-20140730101924

Well, like Sanguinouslives said above, there were genestealers and other Tyranid forces in the galaxy during the era of the Great Crusade. I believe that the Horus Heresy would have been avoided because the Primarchs would be all too busy facing the swarms rather than consorting with Chaos. So with that in mind, the Tyranids would have probably been outsmarted by the Primarchs and the Emperor, and would have tricked them into leaving for another galaxy. I'm sure Magus and the Emperor would have slight issues with the Shadow in the Warp at the height of the conflict, but they'd manage somehow.