If you are looking to make something epic out of the Negative Material plane, I suggest you pick up a few Michale Moorecock Elric of Melnibone' books. There are two swords, Stormbringer and Mourneblade, that are massively powerful soul-sucking longswords that are actually the inhabitants of another plane similar to that of the Negative Material. In the books, just the slightest scratch was enough to take a person's soul from them forever, and the swords could move on their own volition, preferring to eat the souls of those loved by the wielder most of all.
The hook to these weapons is that they would eat MOST of the soul, but give their owner large amounts of soul energy for it. The rest of the soul would be trapped in the sword to spend eternity in agony. Quite the plot hook.
I've incorporated this into my game, with the entities being warriors and nobility of a master race in the Negative Material Plane. They must make pacts to come to a prime material or celestial plane, usually ending in being a Black Sword or something similar, so the effects of the Positive Material Plane doesn't destroy them and they can feast on the life force of the enemies of their mortal wielders.