Nikosen

was a yōkai 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 substantial physical transformation, taking on the appearance of a huge beast with a body made of wood and face of an old man. By slaying Nikosen with the Dragon-scaled Tessaiga, Inuyasha managed to absorb his holy aura and counter the backlash of yōki.

Physical description
Nikosen appeared as a large, four-legged beast with a plant-like body and root-like appendages and tails. His head was still humanoid, with large round eyes, sharp teeth, and a mustache. He had a small hunch on his back from which a new head could sprout, and he could also grow several roots from his body that acted like tentacles. The inside of his body was hollow, and contained a pure white, shining root called the Seimeikan.

Powers & Abilities
Being a former hermit, Nikosen could take advantage of a holy aura along with his demonic powers. He needed to feed on plants in order to preserve his powers.
 * Regeneration: As long as his Seimeikan was intact, Nikosen could regenerate his body parts (even heads) at will. His severed head could still fly and move on its own. However, he also claimed that he needs to "refill" his energy after using this regeneration, which he accomplished by drinking the blood of humans.
 * Root Manipulation: Nikosen could produce large roots from his body and manipulate them like tentacles. Due to the nature of these roots, he could easily move underground.
 * Holy Aura: Since he was a monk, Nikosen had a holy aura, protecting him from demons and other demon-slaying techniques (like Kagome's sacred arrows).
 * Seimeikan (Life Channel): The Seimeikan was a special organ located inside Nikosen's body resembling a white root. As long as this organ was intact, Nikosen was immortal and could heal from any wound. When the organ was severed by Inuyasha, Nikosen died and turned into a pile of wood.