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?