Iron Armour Seal

This is the fūinjutsu that was used to seal the Eight-Tails into Killer B which takes the form of the kanji for on his right shoulder. Not much 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 contained the Nine-Tailed Demon Fox.

Trivia

 * In the manga, the seal is depicted as being black. In the anime however, the seal is blue.