Nikosen

was a demon, who decided to devour Inuyasha and Kagome to guarantee his existence as an immortal. Nikosen was previously a monk, who started absorbing the life force from trees to extend his life and thus stay an immortal. In the process, he underwent a huge physical transformation, thus taking on the appearance of a huge beast with a body made of wood and face of an old man. By slaying it with the Dragon Scaled Tessaiga, Inuyasha managed to absorb his holy aura and counter the backlash of youki.

Appearence
Nikosen appears as a large, four-legged beast with a plant-like body and root-like appendages and tails. The head is still humanoid with large round eyes, sharp teeth and mustache. He has a small hunch on his back from which a new head can sprout, and can also grow several roots that acts like tentacles from his body. The inside of his body is hollow and contains a pure white, shining root, the Seimeikan.

Power and Abilities
Being a former hermit, Nikosen can take advantage of a holy aura along with his demonic powers. He needs to feed on plants in order to preserve his powers.
 * Regeneration: As long as his Seimeikan is intact, Nikosen can regenerate his body parts and even heads at will. The severed head can still fly and move on its own. However he also claims that he needs to "refill" his energy after using this regeneration, which he does by drinking the blood of people.
 * Root Manipulation: Nikosen can produce large roots from his body and manipulate them like tentacles. Due to the nature of these roots, he can easily move underground.
 * Holy Aura: Since he was a monk, Nikosen has a holy aura, which means that he's unaffected by other demon-slaying techniques (like Kagome's Arrows) and that demons can't harm him either.
 * Seimeikan (Life Channel): Is a special organ located inside Nikosen's body and resembling a white root. As long as this organ is intact, Nikosen lives and can heal from any wound. When the organ is severed by Inuyasha, Nikosen died and turned into a pile of wood.