The Shadow in the Warp is a psychic phenomenon cast by the vast hive fleets of the Tyranids as they descend upon the galaxy. Generated by the countless billions of creatures within a fleet and focused through the will of their synapse-beasts, it drowns the immaterium in a smothering psychic roar that severs worlds from the wider Imperium.
Within the Shadow, astropathic communication fails, warp travel becomes perilous or impossible, and the Astronomican itself grows faint and unreliable. Psykers caught in its grip are tormented by a ceaseless droning hunger, and many are driven to madness or death by the sheer alien pressure pressing upon their minds. Isolated and blind, an afflicted world can neither call for aid nor learn the fate of its neighbours.
The Shadow serves as both weapon and harbinger, for its coming heralds the approach of the Great Devourer. By the time a world realises it has fallen silent, the tendrils of the hive fleet are already closing, and there will be no salvation from beyond the dark.