Iron Armour Seal

This is the fūinjutsu used by Kumogakure to seal Gyūki into its respective hosts. The seal itself takes the form of the kanji for and is usually printed somewhere on the jinchūriki's body: Killer B's right shoulder, and Fukai's chest.

Not much else is known about the seal, besides the fact that it was stated by B to be of lower-quality and weaker than the Four Symbols Seal, which was used to seal Kurama inside of Naruto Uzumaki.

Trivia

 * In the manga, the seal is depicted as being black. In the anime however, the seal is blue.
 * In the anime, the Five Elements Unseal appears to be strong enough to break the Iron Armour Seal.