I can safely say it is a Rinnegan Technique.
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Using the power of the Rinnegan, the user projects a shadow of themselves into the invisible world known as Limbo (輪墓, Wheel Grave). The shadow is generally impossible to detect or visually perceive without the Rinnegan. However, Naruto Uzumaki gained the ability to sense its presence after receiving the chakra of all nine tailed beasts and half of the Sage of the Six Paths' power. Madara's shadow was strong enough to repel all nine tailed beasts in a single instance.
Although it is invulnerable to most physical attacks, techniques which make use of the Sage of the Six Paths' chakra can cause harm to the shadow. The downside to this technique is that the user's shadow, after a set period of time, automatically returns to their body and it is left temporarily unusable.
Classification: Rinnegan, Kekkei Genkai, Ninjutsu, Dōjutsu, Clone Techniques