All Hellspawn are entirely composed of the substance known as Necroplasm, it is one way that the otherworldly material of Hell may manifest itself within the real world of Earth. Spawn wears a suit that is itself necroplasm-based and interacts with his fully necroplasmic body, which lives of him for substance. Mainly it feeds of the raw power of Spawn and his Necroplasm. It alters his physical appearance and allows him to use his body as a living weapon.
The suit is a "living" symbiotic organism powered by Necroplam which provides Hellspawn with various powers and great protection. Hellspawn can also shoot beams of green Necroplasmic energy from their body. Additionally, both their blood and eyes glow with the same green light, and also many of their magics and powers, suggesting that raw Necroplasm glows green.
Hellspawn are all created with a limited amount of energy contained within their Necroplasm, each of them have 9,999 units of power to begin with. However, once this energy is consumed, their time on Earth ends and the Hellspawn return to Hell.