Necroplasm is a will-controlled magical substance featured in the fictional Spawn universe. It was originally indistinct/synonymous with psychoplasm, the substance of which Hell itself is composed. It is similar in properties, but special in that it can exist on Earth without needing to remain in one of Hell's spheres of influence as well as being allowed to traverse the mortal world without violating the non-aggression pact between Heaven and Hell. Necroplasm was a creation of the Grand-Mal Demon known as Leviathan and is exclusive to the 8th Sphere of Hell. He used it to create a demon in his own image, known as The Malebolgia (Mal), who turned on his master and usurped the 8th Sphere for his own. This gave Malebolgia full control over the necroplasmic reserves and thus the ability to create a unique army of warriors known as Hellspawn which he uses to tip the balance of power in Hell toward the 8th Sphere.
All Hellspawns in Todd McFarlane's Spawn comic book series (and other media) are entirely composed of this substance. Necroplasm is one way that the otherworldly material of Hell may manifest physically in the real world of Earth. The main character of the series, Spawn (Al Simmons), wears a suit that is itself necroplasm-based and interacts with his fully necroplasmic body. It alters his physical appearance and allows him to use his body as a living weapon by endowing him with superhuman powers of strength, bodily density and weight, healing factor, and metabolism. The suit is a living symbiotic organism which provides him with various powers and great protection. He can also shoot beams of green necroplasmic energy from his body. Additionally, both his blood and eyes glow with the same green light, and also many of his magics, suggesting that raw necroplasm glows green.
Hellspawn are created with a limited amount of energy contained within their necroplasm, measured as 9,999 units to begin with. Once this energy is consumed, their time on Earth ends and the Hellspawn returns to Hell, his soul forfeit.