Steel Release

The is an advanced nature kekkei genkai that can be used to make one's body virtually indestructible by turning it into black steel. The elements combined to create this nature are unknown.

To date, this form of nature manipulation has only been used by Hiruko, who obtained it using his Chimera Technique to steal kekkei genkai in Naruto Shippūden 3: Inheritors of the Will of Fire from an unknown shinobi.