Soul Symbiosis!
This was Uchiha Yue's trump card!
Why had Uchiha Yue's Rinne Sharingan continuously regressed during his earlier clash with the Sage of Six Paths? The reason was that Uchiha Yue had used a forbidden secret technique, drawing upon the Rinne Sharingan's ocular power to wield a time-based secret technique!
Before obtaining the Rinne Sharingan, Uchiha Yue had understood very clearly where his strengths lay. Thus, while painstakingly cultivating the ocular power of his Eternal Mangekyou Sharingan, he had also been researching its secret techniques. Asura Susanoo went without saying—it was one of Uchiha Yue's trump cards. But his other trump card was a time-based secret technique that no one in the Shinobi World could master, one whose mysteries even the Sage of Six Paths could only glimpse!
Therefore, when Uchiha Yue unleashed all the ocular power of his Rinne Sharingan, it regressed with the depletion of that power, slowly transforming into an ordinary Sharingan. At the same time as the Rinne Sharingan regressed, time itself rewound under Uchiha Yue's control. By unleashing the Rinne Sharingan's ocular power, Uchiha Yue had actually sent the world controlled by the Sage of Six Paths directly back to the moment when the Sage of Six Paths had just been born. The terror of Uchiha Yue's trump card was beyond question.
However, by the time he arrived at the moment the Sage of Six Paths had just been born, Uchiha Yue was already extremely weak. Especially after studying time-based secret techniques for so long, Uchiha Yue knew of one flaw in them. Now, standing before the infant Sage of Six Paths, the Body Energy within Uchiha Yue's physical body had been completely exhausted. In other words, his body had already decayed, and all he had left was the energy of his soul—Spiritual Energy.