Steelborn Iron body
The Steelborn Iron body is an Iron body focused on strength, used by Yerin Arelius and her master the Sage of the Endless Sword. Ruby also gains the ability to use the Steelborn body to an extent, though it is weaker because she doesn't have a real body. The Steelborn body allows its user to channel their madra into it to improve their physical power. Yerin describes it as being "the best Iron body in the world for pure brute strength" on advancement to the Lord realm, though it is relatively weak before that. The Path of the Endless Sword does not have a very good full-body Enforcer technique, making the Steelborn body a good choice to cover for its weaknesses.
It is attained by being submerged underwater, breathing through a reed, for some time prior to advancement. Like other Iron bodies, the longer the ritual is undergone, the greater its effects will be for the user; Yerin spent three days and three nights in a pool in preparation for advancement.