"They had teleported into the Red Angel's Gate. The Grey Knights stood on the very threshold of the Sanguinary Unholiness' infernal domain. Physical law and mental reason no longer applied. The 'sky' had an unnatural curvature that extended far beyond the natural point where a horizon should have intersected it. It was jagged, faceted, often bleeding – a broken mirror holding up a million fractured images of Armageddon's warped surface. 'Ground' was a term of orientation rather than of substance. It was what lay beneath them rather than above, and appeared no more substantial for all that it bore their weight.
The unnatural geographies rang with the screeches of things that had no lungs and felt no pain. Lithe, red-skinned Neverborn raced across ground and sky and the space between as though they were one and the same. As though it were raining nightmares. Some were winged and armed with spears. Others rode on thundering monstrosities of brass, loped after by fearsome scaly hounds in spiked collars that bled the very essence of pain. The insane topography would have shattered a mortal's mind into more pieces than the sky above. Even a Space Marine, venturing into such a realm unprepared, would have found himself crippled by an existential nausea his gene-perfect physiology was ill-equipped to abide.
But this was the battlefield for which the 666th Chapter of the Adeptus Astartes had been founded."

-Angron: The Red Angel