Plot Idea: Revenge Party (Fantasy/Supernatural)

The previous queen to a small nation next to the empire married the king for love, but was unable to have children. The king still insisted he needed an heir, and so an illegitimate child was born, and was made the heir to the throne. This prince grew up as the child to the queen, however she died a few years later due to a heart problem. The illegitimate prince was devastated, and the king claimed he was as well, but then remarried a mere two months later saying it was for the good of the kingdom. As the prince grew up, the royal palace realized he wasn't good at anything really, except maybe making up schemes and hiding his misbehavior by blaming other people. The king was very disappointed in him, yet he had already declared him a heir. That was until the empire was found to be preparing to take over the whole continent.

The illegitimate prince's only sibling was his two year old brother, however once the kingdom was threated, the kind stripped him of rights to the throne under the excuse he had to earn it in battle. The illegitimate prince was not know for his ability to fight, and was basically sent to die as a general. The prince, ever the schemer, however, decided he was not going down without a fight and searched for something to aid him. He then found a book hidden in the family treasury on how to summon demons, and decides to at least summon a mid-level demon that can kill his father and the current king while he is off at war. Instead, he somehow summons a demon lord, one of the most powerful demons in existence. Intrigued she was able to be summoned, the demon makes a deal with him.

As long as she could take one human soul a month, she would help him do more than just kill his father as the prince died. She would help him defeat the empire and come back to the kingdom to get revenge of every single person that ever wronged him.

Notes:
-Gender can be swapped for the prince
-Can add romance if wanted