Mōryōmaru

Moryomaru (魍魎丸, Moryomaru) began as a Buddhist monk that watched over a shrine deep in the mountains. He took care of many orphans after the nearby village was destroyed. One reason he hid in the mountains was of his "grotesque" arm, a youkai arm that he acquired protecting the shire some time before. The arm has no evil aura around it and seems peaceful. InuYasha and his party were suspicious of him at first but later trusted him. Come to find out their initial suspicions were proven correct, Moryomaru was working with Naraku and was hiding Akago, a.k.a. his heart, and was revealed to have in fact merely been a number of youkai merged together, which also has the power to absorb other youkai and gain their powers, as a body Akago used for his protection. He escaped from InuYasha's grasp and continued to grow stronger, disobeying Naraku again and again.

To aid his plan of growing stronger, he revived a turtle youkai called Meijou, which went on a rampage in effort to eat metal and recreate its shell. When Meijou completed its shell, Moryomaru absorbed after a failed attempt by Inuyasha to kill it with Kongosoha (though the spears from the attack still stuck to the youkai's shell), giving himself a nearly indestructible body, and the ability to use Kongosoha's diamond spears (which he absorbed along with the Meijou) as weapons. He became nearly invincible, overwhelming even Inuyasha, Sesshomaru and Kikyo, and was only driven off in his first fight with them when Sesshomaru striked with Tokijin with such force that the sword broke. He became so strong that Naraku had to go as far as to aid InuYasha so he could weaken him. This seemed to to nothing in the end because recently Moryomaru just devoured him. However, it later turned to have all been part Narku's plan, as he absorbed Moryomaru from the inside out.