One Piece

Tekkai hardens the user's muscles and skin to the level of iron, significantly boosting physical durability and enhancing attacks. While active, the user cannot move, but can withstand physical damage up to their strength limit, beyond which they may be injured. Tekkai does not protect against non-physical attacks, such as fire or magic, making the user vulnerable to certain threats. Mastery of Tekkai increases its effectiveness and the user's resistance to damage.