Astrum (歪界(アストルム) Asutorumu; "Distorted World") is a realm parallel to our universe, and the origin of magic in the universe of the series. Astrum serves as the homeland of Diva, supernatural creatures which can be summoned through Enigma, mysterious marks appearing on the skin of a human (usually, but not always, female) chosen to interact with Astrum. In a way, people possessing the Enigma act as messengers between the world of Astrum and our own.

Not long before the start of the novel, the "wall" between Astrum and the human world was cracked, allowing the former to start influencing the latter. A short time after this, countries all over Earth formed theocratic societies worshipping the Diva, alongside Enigmas appearing seemingly at random.

According to Lemegeton, Diva cannot remember the events that happen in Astrum, although they carry the memories of the time they spent in the human world back to Astrum.

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