Six Paths Yin Power

The Six Paths Yin Power is a tattoo that was used by Hamura and Hagoromo Ōtsutsuki, and later given to Sasuke Uchiha.

Background
Besides being comprised of the Yin Release nature transformation, the actual applications of this power aren't clearly understood. What is known, is that it enabled Sasuke to infuse his Chidori with the Six Paths power, allowing him to bisect Madara Uchiha as the Ten-Tails' jinchūriki.

As noted by Black Zetsu, there is a special resonance between the Six Paths Yin and Yang powers, allowing the wielders to sense each other's presence. When the Yin and Yang powers are used together, they can transform their target into the core of Chibaku Tensei. When Sasuke did so in conjunction with Naruto Uzumaki's Yang power, the mark on his left hand vanished, returning to Hagoromo's own left hand.