The story was quite simple

There's a king who rules his nation.

The king does not elect a queen and have many wife instead

It resulted the king having many son and daughter (the king does not have any crown prince )

The king was quite scary , and tyrant, so nobody oppose him

One day the king tell all his son

There's a demon in the forest, whoever managed to slay him, will become the next king

Thus , many of his son challenged the demon and died

Nobody dare to challenge the demon after that

The story then shift to two prince and their sister(still the king son/daughter)

The two prince was around 14-15 years old and the sister was around 12 years old

The two prince tells the tale of the demon to the sister to scare and prank her

They live happily till one day the sister got illnes that can't be cured

The two prince then heard, the demon will grant any wish but with some payment

The two prince go to the forest, fight with the demon but lost and plead to save their sister and they will pay it with their life

the rest of the story sounds vague because i forgot the story