The Nothing (La Nada) is a cosmological concept: what remains when everything else recedes. It is embodied by Erebus.
It offered the elves of the Forest of Black Poplars the chance not to shrink before growing humanity, poisoning their home and souls with the essence of the Shadow. As the Shadow advances, the tall poplars spread “amid the void of what remains.”
Instruments such as the Rod of the Nothing, and places such as the Pillars of the Nothing, descend from it.