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The Gaetia Key is an artifact to be used by the Gazel Ministry to revert humans back to their original forms (Wels) via gene manipulation. It was to be used at the time of Deus' resurrection, in order to convert humans to "replacement parts" for Deus to absorb in order to rebuild itself.
It isn't entirely certain how the Gaetia Key released a lock that mutated the rest of humankind into their true Malakh forms. It may have changed the composition of the planet's atmosphere, adding a gas that activates a recessive gene in Deus-based humans, or perhaps activated a certain Ether.
The Japanese words say Lesser Goetia Key, a reference to the Lesser Key of Solomon, an anonymous 17th-century grimoire, which is also known as The Goetia.