Now Something a Lot Darker - The Monster Under Your Bed

I thought about posting this earlier, but it was long enough (and much darker than previous ideas) that I thought it should have its own ad.  Since I don't want to risk having too many ads at one time, this may be the last new one for a while.

The concept here is a story that starts with a monster and human girl, though that will change over the course of the story. Fair warning again, that it gets rather dark.  Hopefully I am right in my understanding it isn't too dark for the site.

One way or another a monster ended up under the girl's bed. How did it get there? Portal from another world/dimension? A unnoticed egg mixed with other stuff put under there? It is a fairly small detail that does not have to be considered but we can figure it out if you want to. The monster quietly grows under the bed and waits, sneaking out to eat at night. The girl may or may not notice it, but if she does she is too afraid to try to get rid of it. Until one night.

The exact nature/shape/features of the creature can be discussed. But one night, once they are both old enough, it comes out and onto her bed. Maybe it is has a stinger, maybe it bites her, one way or another it gives her some sort of toxic relaxant. She is not put to sleep or immobilized, but it relaxes her to the point she does resist at all as it mates with her. She does not care, maybe even enjoys it (at least for the night, while its happening, when she wakes up she is likely not nearly so happy about it, but she might be also confused as to whether it was a nightmare or real).

What she does not know at this point is that was only the beginning. Rather than inseminate her, the mating released a fluid into her that slowly begins a process of changing her. It takes a while, and possibly multiple mating sessions, but eventually her mind and body start to change. She first begins to take on physical aspects of the creature, the initial changes can possibly be hidden with varying degrees of difficulty, but eventually she will have to abandon her current life.

As more of her body changes, she becomes somewhat more comfortable with the creature and it no longer gives her the toxins. Instead it offers her raw meat from his hunts, encouraging her to eat it. This speeds up the physical changes and starts the mental ones. She feels uncomfortable around other people (which was already somewhat true due to the changes she had to hide, but now it is more hostile than scared). Eventually she snaps and kills someone. That is when all the pieces comes together and she figures out more of what is happening. Her mate had been feeding her human meat. His kind, and what she is becoming, are predators that feed on many things but often crave humans. Possibly with some lingering shame and humiliation, she eats the person she just killed.

This is when the mental changes really intensify. At some level she remembers she used to be human, but no longer thinks of herself as one, no longer thinks like one. They become prey and she loses any remaining shame about hunting and feeding on them. At this point she has to find a new place to live, and the male returns, to lead her to a place he had in mind. At this point they mate again, this time her body fully changed, and the purpose becomes to have children, which they both want at this point. 

At this point things can go a few different ways. Maybe these creatures mate for life. In which case their 'relationship' is not something we would recognize as love, but act as a couple of sorts and work together and keep having children, more similar to animals who mate for life. Another option would be that the first male's role is done after his first child/children are born and she has to seek out other males of her new kind maybe once a year or so. I like the idea that she can only give birth (lay eggs maybe) to males and females have to be created the way she was. They are creatures who are unique in the world but due to very small numbers and secrecy, have not been discovered yet.  It could continue on to explore the lives for such creatures and the sorts of things they might encounter that humans know nothing about.