In 926.M41 the Purge decided to make the pretty Imperial world of Vaxhallia their next stop in their quest to kill everyone and everything ever. Showing up out of nowhere they proceeded to drop every kind of space daemon aids imaginable onto the bitch. Most of the planet was a graveyard by the time the Imperium responded. You see the planet was actually an Astropathic hub and fortress planet vital to the sector‘s defense, so when the Imperium got there it was primed to get shit of its own done, with the whole Hammers of Dorn, Fire Lords, and Crimson Fists chapters, a dozen guard battle-groups showing up, along with a few Knights from House Raven, just for good measure.
In response Typhus showed up with even more Plague Marines and a boat-load of zombies. Both sides stared each other down and an epic battle began as both strove to get the most shit done. In the end the Imperium lost as more and more of their own guard forces were turned into plague zombies. Oops. When the second Imperial force arrived the commander recognized the planet was screwed and began planning to evac the survivors and blow the planet to hell. This would of course kill anyone remaining on the planet and result in even more death, which was what the Purge wanted all along. Just as planned. Bet they wish they had ships to just firebomb planets without outright exterminatus, eh? That's what you get for not using enough fire!
That said, the idea of such a critical world being lost to anything short of an entire Legion and its fleet and army support is pretty hard to believe. Especially since defending something critical to a whole sector is something a Legion-strength force should be sent to deal with.