Thunder Mask

The Thunder Mask (King of Light in the Light Novel) is a giant being made of light. It's a serpentine-like appearance in the anime with a red and white mask on it. In the Light novel, it's a humanoid wearing a crown of light and wearing royal clothes.

Appearances
It appears as a boss in the Fifth Floor. Depending on the version (Maple in the Light Novel, Mai and Yui in the anime) the three maxed stat players couldn't defeat this boss by themselves. Only by working together did the three of them take down this boss. Mai and Yui got skills that let them use lightning while Maple got the skill [Throne of the Heavenly King].