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The Six Paths Yin Power is a power wielded by Hamura and Hagoromo Ōtsutsuki, and later given to Sasuke Uchiha. It is represented by a dark-colored, crescent moon-like marking on the user's left palm.
Besides being comprised of the Yin Releasenature transformation, the actual applications of this power aren't clearly understood. It can be presumed that it augments the user's abilities.
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 seal and transform their target into the core of the Six Paths — Chibaku Tensei technique. When Sasuke did so in conjunction with Naruto Uzumaki's Six Paths Yang Power, the mark on his left hand vanished, returning to Hagoromo's own left hand.