Comment by Hakutou on A Fortune-Telling Princess - Chapter 26

When the ML and his younger brother was young, their father locked them up in the basement, and they will not be allowed to get out until one of them dies because it is needed for the succession of the duchy. So 3 months or so after, the ML's brother died and the ML got out and resented his father because of what happened.

Since the FL is able to talk to ghost, the FL met the ML's dead brother. His name is Sierre. Sometime after, they learned the truth about why they were locked up in the basement. The ML's father kept beating him everyday and Sierre is actually sick and only has a few months to live. Sierre, then decided to go to the basement with his brother to help him in the succession of the duchy through his death.

Turned out that to be able to get the divine beast of their family, the successor need to kill the previous duke and get the divine beast's egg in his body. So the father kept beating the ML so he would resent his father and be able to proceed with getting the divine beast's egg without hesitation. And Sierre's death will stop the beatings because his death will be more than enough to resent the ML's father