Fantasy novel where in the end, the protag and few others venture to an abandoned kingdom/castle only to find a giant snake monster who hunts them down but another snake monster awakens and helps the protag.

[Arya]

There are also occasional beams of light from the sky and the protagonist seems to gain some sort of abilities at the very end. Also, the protagonist is in some sort of a school.

[Arya]

The abandoned kingdom/castle had a lot of engineered creatures out of which only the snake moster had broken out and attacked anyone who went there