It took me a little while to get into this, the first section is slow and heavy laden with grief. But I'm glad I stuck with it because it turns into something very different. This is, after all, one of the common horror tropes, beginning with the death of a child and then following the parents in mourning. It is about change, about becoming a different version of yourself, about finding the world you are fit for when you're not fit for the one you're in. And, of course, it is about a little monster.
The book moves through several points of view. We begin with Magos, the grieving mother, but whose grief does not present as sorrow. She comes back to her home in Mexico City because her husband, Joseph, struggles to be around her when she doesn't seem to share his sense of loss. Magos will bring the little monster into the world, fueled by her own strange suffering. There is Lena, Magos's closest friend who has also spent her whole life in love with Magos. And when Joseph returns, we see that he and Magos are not the same people they once were. Their loss has changed them, and so has the little monster.
The monster himself is exciting. He doesn't fit any existing model of monster, or at least not any that I've heard of. He also does not stay the same. He is unpredictable. Magos believes she knows what he will become, but how much of this is Magos' desire for her lost son? How can Magos and Joseph see the monster without that lens? He is, eventually, known as Monstrilio or M, and as he changes he forces those around him to change with him. And they do, until they reach the outside boundaries of what they want M to be and what M wants for himself.
If that sounds not very horror-y, well, it often isn't. Though there are some quite grisly scenes. (This includes scenes that feature kink, both with and without consent.) It is the kind of book that should satisfy many more "literary" readers (yes I am rolling my eyes a little, but the point remains). For horror readers it may move too slow for some, but if you are looking for something unusual and new that doesn't feel like anything you've read before, which I absolutely am, Monstrilio fits the bill. A great debut.
Also was surprised to discover around halfway through the book that a majority of the main characters are queer! The book itself felt very intwined with themes of parent/child as well as queerness, especially in M's self-discovery.