Back when the entrance to the Netherworld from the outside was not sealed, very few people appeared at the Divine Battlefield. Even if they did appear, it was mostly at night. During the day, the sunlight would cause severe harm to their bodies and limit their powers. As such, the Netherworld Lord's clone only possessed the strength of the Sixth Level of Holy Martial Realm.
The Sun Shining Stone can emit light akin to the sun, causing great harm to the people of the Netherworld. Chu Linfeng had forgotten this detail when dealing with Xiang Shaolong and only remembered it after thinking about the Netherworld Lord's clone; otherwise, he wouldn't have fled during his confrontation with Xiang Shaolong.
