From the introduction
First truly encountered in the latter centuries of the 41st Millennium, the Tyranids are one of the gravest threats ever to face the galaxy. Utterly relentless, knowing no fear or doubt, they are driven solely by an insatiable hunger, forcing vast hive-ships from world to world, consuming all life and leaving nought but dead planets in their wake.
This supplement for Warhammer 40,000 Roleplay: Dark Heresy is completely unofficial and in no way endorsed by Games Workshop Limited.