Demiurge - Half-Demon Form

Dimly feeling something akin to loneliness, Momonga heard the sound of flapping wings—Demiurge had transformed.

Black wings made of some kind of moist-looking membrane had sprouted from his back, and his face had turned from a human one into something vaguely frog-like. This was his half-demon form.

Some grotesques had multiple forms. In Nazarick, for example, Sebas and Albedo had other forms, too. Those types of grotesques took some trouble to make, but they were consistently popular because people enjoyed having multiple forms like a final boss. Many of them were set up so they took penalties in human or half form but received bonuses in their full grotesque form.

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Volume 1, Chapter 3.1