Black Dragon
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Before Monica had begun to use magic abilities in real life, she was someone that harbored a large quantity of negative energy and cursed mana within her. All born from the constant bullying she received back in her original school. The truth is, that although she harbored a lot of such energies thanks to the bullying, Monica was, from the beginning, more attuned with Mana than the rest.
No normal person would ever be able to harness so much cursed mana and negative energies inside of their bodies, hearts, and souls otherwise, which also made her the prime vessel for one of the Demon King of Death's strongest servants, Dorothea, the Princess of Nightmares, an incredibly powerful specter with the ability to create entire Realms of Nightmares and Dreams, where she had trapped Monica and the rest of the students, aiming to consume their souls.