We can see that light. Those of us within the Empire of the Eye can actually see it. The Astronomican reaches even to our purgatorial exile, and to us it is no mere mystical radiance illuminating the warp. It is pain, it is fire, and it plunges entire Neverborn worlds into war.
It would be a mistake to believe the Emperor’s power battles the Four Gods’ forces, here. It is not order against chaos, nor anything as crude as ‘good’ against ‘evil’. It is all psychic energy, crashing together in volatile torment.
Most of the Radiant Worlds are uninhabitable, lost in the lethal crash of conflicting psychic energies. Armies of fire angels and flame-wrought projections wage war against everything in their path. We call this region the Firetide. What made the Avernus Breach so valuable was its path, not its destination. It cut through the systems forever bleached bare of life by the Firetide, and into the calmer Radiant Worlds beyond. These are the star systems bathed in psychic light without burning in it.
Entire centuries will pass without a single vessel sailing the region, for it offers little to us beyond yet another example of soul energies manifesting in ways mortals can barely control. On more than one occasion the Mechanicum has sought to use Neverborn spirits bound within arcane flesh-machinery to record the Radiant Worlds in an ever-shifting, evolving map. Such attempts have fared as poorly as you might imagine.