Siegfried

The London Siegfried (ロンドンジークフリート - Rondon jīkufurīto) or better known as simply: "Siegfried" is the antagonist of the Rebirth Story, descending from the sky on Walpurgisnacht to consume Walpurgis and destroy the earth, enacting one of the titular "Rebirths" that the characters are trapped in.

Not much is known about it, or what it truly is, but it is often referred to as a "he" and "unborn", suggesting that a creature of such monumental power has yet to even be born, and is merely preparing for something greater.

It is worshiped as the son of God by the Black Moon Cult, and is key to the plans of the Fathers and the overall goals of the Blue Oyster Cult. The Cult of Gretchen has taken interest in the Archfiend as well, saying that it will bring about "heaven" or some form of "paradise" once only capable by the Witch of Salvation herself- The Archfiend being her "gift" to humanity.

Whatever it's true purpose is, it must be stopped in order to prevent the world from being trapped in an endless cycle of death and rebirth, or perhaps a fate far worse than that.