The timeline is when Xia Youjie and Gojo Satoru were protecting Riko Tenuchi. Because of the failure of protection, Xia Youjie was severely hit, but Gojo Satoru awakened to the magic reversal of the technique in the battle with Zenyuan Shiji. The two gradually drifted apart from "we" being the strongest to Gojo Satoru being the strongest...
Purpose: To bring the spell to a better ending