The World Engine (Space Marine Battles, #15)

For months, the World Engine has blazed a trail across the Vidar sector, destroying planets and devastating every fleet sent to destroy it. Now, the Astral Knights Space Marine Chapter enact a daring plan to get to the heart of the mighty edifice and bring it to an end. Crashing their battle-barge into the World Engine, they land upon its surface, seeking its heart. Confronted by sinister necrons, the fate of the Astral Knights hangs in the balance, along with the lives of untold billions...

Read it because
You can't beat a good super weapon, and they don't come much more apocalyptic than the World Engine. It's not often we get to see a full Space Marine chapter in action, but nothing short of that will be enough to stop this planet sized ship - and even that might not be enough.