Out there somewhere exists a magical forest that has interesting effects on humans. If a human finds themselves in this forest at night, they transform into a werebeast. There is no telling what they turn into, but after that first transformation, they are bound to the forest for the rest of their lives.
After the first night they turn back to human and can transform different parts at will, but will be forced to transform fully on nights of the full moon and on those nights, all inhabitants find themselves incredibly horny. Any resident of the forest can find themselves pregnant and will know if they are when they can't transform back into their human forms. Unlike human pregnancies, the child is born in less than a month and the person who was pregnant can become so again almost instantly.
Any child born under these circumstances are also bound to the forest and will start undergoing puberty as early as ten years old and as soon as they are finished will transform for the first time under the next full moon. Because of horny the residents can be, incest is acceptable. However the chances of a child being born through incest are less than 5%. Even then the child will be born without any defects.
In the event that a human has some sort of illness or disease, they will no longer afflicted by them after their first transformation. Their lifespan is also increased drastically to around 1,000 and for the first 200 years of their life they will have the sexual drive of a young adult.
There is also a high chance they will gain a good amount of muscle on their human forms after their first transformation.