Gate IS a Nightmare.

Black smoke rises from below Dark Gate, and he begins to get taller, stronger and grow a horn. At the time this is going on, the darkness overpowers the real Gate, and he is faded to nothingness. Dark Gate now has dark grey fur, massive wings, his heart necklase has turned into a large black pendant, "Hahaha! What should I call myself... Nightmare General, that'll do."
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You've failed, Gate was defeated, and Dark Gate won.
I'm sorry.