The five senses of mortal man cannot percieve the Devourer, for it is not a part of the reality of Man. The Devourer lives in the bizzare realm-that-is-not on the other side of the weft of space and time, a place that Man cannot go, nor percieve, nor even imagine. However, it is not without its effects on this side of reality, for wherever it passes, gravity changes strangely, as it presses back against the nothingness that births it.
Excerpted from the Journals of Ralthok the Shaper :
“Ye gods of Darknesse… what have I wrought? E’en in the grip of the visions from the Desert Bloom’s elixer, such a simple thing should not have gone so awry. To call a minor thing such as a succubi, and create this… the gods of Blackness must have chosen to shape this. Surely it cannot be my fault. The hunger of the elixer cannot possibly be sufficient to warp my casting, not so much that my laboratory itself is swallowed, along with my cohorts!
My grimoire without it, I can do nothing. I must flee, and hope another can stop this… thing.”
Hallucinating and hungering from the effects of a semi-magical drug, Ralthok’s summoning ritual was, indeed, warped, his own wild set of mind combining with the unnaturally awakened thoughts of his friends to weave the Devourer.
A creature of indeterminate shape, the Devourer lurks on the outside of the universe, and when it hungers, it eats the only thing available for it to feed upon - reality itself. Whatever exists where it chews upon is lost to the Devourer, and a ragged hole is torn in the naturally folded fabric of reality. Abhorring this, reality swiftly, but not instantly, seals against itself, two places becoming, for a time, one, and the sated Devourer is shunted aside, to another place. When one steps across the line where the Bite of the Devourer begins, one is seamlessly transported to wherever reality has sealed against itself, for there is no space between it. In time, however, this ‘scar’ upon reality heals, the bitten space peeling back away from the merged place, though now, the space between the marks of the bite is gone, as if it never was at all. Any matter at the site of the bite also winds up on the ‘underside’ of reality, where it may or may not be able to continue to exist. It is most likely it will evaporate into nothing, upon a pyre of inverted physics.
To effect the Devourer with conventional magics or most technology is entirely impossible - to reach the Devourer, the effect must be able to strike through the space time itself. This limits to dimension-effecting spells, gravity effects, etc.
The Devourer has no intelligence. It is simply occasionally hungry. It’s ‘bite’ may be variable, from man-sized, to city sized, depending on the needs of the plot.