Predator, Prey

After centuries of peace, the Imperium is thrown into panic as human worlds everywhere are menaced by orks. In a relentless tide of slaughter, ork attack moons destroy planet after planet with gravity weapons of unstoppable power. On Terra, the High Lords are paralysed by the scale of the threat, and fail to take any effective action. With entire Space Marine Chapters missing, or known to have been wiped out, does anyone have the will and the power to rise to the Imperium’s defence?

Read it because
It's a snapshot of the battles that rage across the Imperium as the orks attack in force. It also continues the story of the High Lords as they try to hold the Imperium together... as long as it suits their own interests, of course. And it introduces characters who're going to be very important to the future of the series... if they survive the battles to come.