Armor of Achilles

A small amulet crafted with a unique Kaguya seal. A portion of Muzai's chakra has been entrapped within. Whenever it comes into contact with Yami's chakra, it either sends out a expansion or contraction command. Expansion causes bones to externally appear around Yami Nara. It begins from the necklace and splits, forming both upwards and downwards simultaneously. Unlike Muzai's Armor, which is light, durable yet strong enough to break metal, Yami's Armor of Achilles is simply heavy. It is not for fighting, rather than stationary defense. Heavy and capable of covering his entire body. Yami's Armor of Achilles fits perfectly with his Shadow Binding. The amulet is not the armor itself. Instead, it is a base for Muzai's chakra to react and grow from. Similar to a wildfire, his armor needs a Kaguya's chakra to stimulate the dormant Osteoblast cells and osteoclast cells. Yami's chakra acts as a button to disturb it. Expanding causes Osteoblast cells to form bones rapidly. While contracting causes the opposite. The amulet reabsorbs the bone matter back into it for future use.

Should it ever break, Muzai can simply repair it with a touch. Also, Yami's armor has a unique ability to regenerate with each use. Each time it is contracted, and expanded, new cells create new bone matter.