The Unmade is what the cancellation at the Caves of Nykthos left behind and never finished. When a shaper of Nilus unmade the cave's own leyline signature, the negation caught a living thing in the lowest chamber and ran partway through it before it stopped. Where it passed, presence simply ceased to be there. What survived is a being whose spirit, the self and the name it once answered to, was half-replaced by absence: a will made partly of nothing, holding to a body that is a little less present with each passing year.
It does not hunt and it does not roam. It is bound to the canceled point and cannot leave it. Its reach is the quiet that spreads around it. Sound will not carry near the Unmade. A lamp brought into its chamber finds nothing to feed on and goes out without smoke. People who come too close lose hold of small things first, the word they wanted, the reason they came, and then larger ones. Expeditions that reach the deepest reaches come back lighter than they went in, packs and names and stretches of memory gone, with no tear or theft to account for the lack.
Whether the Unmade can be killed is the wrong question, because it is most of the way dead already and cannot finish. A living being is carried out of life on three roads: the soul to the Astral, the shadow to Malstaris, the spirit to Celestia for as long as its name is spoken. Freezing locks those roads shut, and a destroyed frostwalker is held whole and stopped where it falls, everything it was sealed in the frozen flesh. Nilus did the opposite here. The cancellation did not close the Unmade's roads; it unmade them, and unmade the substance of the strands that would have walked them, so that nothing can depart because there is nowhere left to depart to, and nothing stays entire because less of it is present every year. It does not stop. It lessens. What remains grows thinner across the centuries and never quite arrives at nothing, an un-being that cannot complete its own ending.
This sets it apart from the Void Walker, the void's native form, which flickers things cleanly out of the world and is whole in itself. The Unmade is the bound degree rather than the native: an ordinary creature with an ordinary death ahead of it, until the working took both the death and the creature halfway and left the remainder stranded between.