In the cold depths of space untold numbers of xenos creatures intend on devouring the worlds of Man have started moving. They make ready to invade the Imperium.
The full rules of using Tyranids can be found in Codex: Tyranids. This is an Army book for Warhammer 40,000 containing complete details for Tyranids. Within its pages you will find an army list, background, new wargear and special rules pertaining to how they act on the battlefield.