Ultraman Nexus: S1 E24 - Hero Ultraman Nexus

Ultraman is defeated and captured. Mizorogi arranges a ritual to steal Ultraman's light from Himeya, and when the Night Raiders attack to try and stop it, he becomes Dark Mephisto. As Himeya is fading fast, he's greeted by a vision of Sera, who shares with him the reason he was chosen by the light...