I was expecting a clash between light and dark when you mentioned it as the theme which could lead to deeper story but I digress.
As for feedback, I believe you should find decent reason or rather what exactly you mean by fallen into darkness. The answer will define what exactly player would do to return light to that kingdom. Old games usually simplify this by the king has fallen to dark side and summoned demon which caused the darkness so player "only" need to defeat the demon to solve the problem. But you could change that, here are what I have in mind ATM:
1. The darkness was caused by multiple person doing evil deeds which brought darkness. Player must stop the evil deeds in multiple places to solve this.
OR
2. The darkness was caused by ancient devil who opened portal of darkness in a kingdom. Player need to seal the portal and defeat the devil to expel him back to the darkness.