A Bedbug Devouring the Wizarding World

The bargain Seth strikes in the magical world is with the Infinite Devouring Evolution System: what he devours, he becomes. He needs it, because he arrives there as a bedbug. Before that he was the king of the mercenaries in North America, and he was shot dead. Scarlet is a witch her own people call a dud, with nothing they count as real magic. She and the bedbug make a pact. He eats dark wizards, then magical beasts, then gods, and each one moves him up a form, as far as the Purple Crystal Sovereign Dragon. The two of them tear down the Supreme Temple and beat the Grand Inquisitor. Seth gets a human body back and leaves for the stars with Scarlet. One of the things he swallows along the way is an aura that had been stolen from the protagonist of that world.