Up to the bones (Tomco)

This is originally a book in spanish by Shamsia on Archive of Our Own.
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"Young Marco, there are two things you should know: First, the Red Moon has affected you in ways I had never done with any human, and second, it has also affected my son Tom." With an unexpected visit from a young demon in his own room, Marco finds that the surprises had only just begun. And I did not know if he liked this surprise.