Hive Fleet Jormungandr was the reason the planet Ager Formosus was rad bombed to begin with. It is only centuries later that the Imperium of Man could re-establish on this once paradise turned death world. Once beautiful beaches are now radiation soaked deserts, majestic woodlands are now acid rain soaked forests twisted and dangerous. The once proud Imperial cities with names forgotten my time are nothing but ruins.
Now that the radiation levels have dropped to allow mining colonies to return, so too have the Tyranids they were used to destroy. Perhaps the radiation exterminatus would have succeeded with any other hive fleet, but Jormungandr burrowed deep. Deep enough to escape the power gamma death that killed everything else.