By the way I've seen some people talking about darkners absorbing human souls to become lightners and it gave me a thought.

Maybe that's what all monsters are? Former darkners? Maybe lightners were humans by default at the start but when some darkners absorbed human's souls they became lightners but kept their inhuman forms? Resulting in monsterkind? That doesn't mean every monster in the game right now has absorbed a human soul because lightner monster parents will have lightner monster children. In other words the monsters in the game all have an ancestry that started with some darkner absorbing a human soul.

Maybe that's what's revealed in one of the last books in the library when we finally get access to it.