The Divine Mirror (神鏡, Shinkyō) is an ancient relic which drives the plot in Warriors Orochi 3 Ultimate. Its function is based on the Chinese belief that mirrors ward evil spirits by reflecting their hideous appearance back at them.
The Divine Mirror notably has its own realm and version of reality containing duplicates of the real world. Notably, this also includes the Mystic Realm and its denizens. The Heavenly Emperor used the Divine Mirror to seal demons away, but the mirror was damaged when Yinglong was tricked into seizing and attacking it by Tamamo. The crack causes miasma to release over Yinglong and transforms him into Orochi. The mirror would subsequently be stolen by Tamamo and was sealed alongside her inside the Killing Stone. After being freed by Da Ji, Tamamo would use the mirror to seal heroes of the Coalition before sending her tails out to impersonate them.
Eventually, the Coalition members inside are able to group together and escape the realm through its cracks at Wujun. When Tamamo escapes once again and cannot be traced, the Coalition would decide to return back to the Heavenly Emperor's time to retrieve an undamaged mirror to seal her permanently. Although the mirror is still damaged by Yinglong, the successful rescue of the Heavenly Emperor allows him to repair the mirror for the Coalition, and they are able to finally defeat Kyūbi and seal her within.