As mentioned in the past, the Light Devils exist in the Underworld to keep a balance between the Underworld and Heavens. At the same time, there are Dark Angels in the Heavens, too. Mephisto's group then attacked once again, but they couldn't get the advantage.
The Underworld Relic is the treasure of the Underworld, used to help protect it. In the hands of a Peak Stage Elder Devil, it showed a power that could fight several Archangel at once. It's just that this power only worked in the Underworld, so it would be useless to bring this Relic anywhere else.
However, Rui's opponents aren't Archangels, which was the original intention of the Relic. They were Devils, so the power the Relic could display wasn't as effective against Mephisto's group. Nonetheless, the fact Rui could fight three Elder Devils at the same level as him already showed how powerful the Relic is.
