Arthur is your older brother, next in line to become a King but due to some complications he has to fight for it.

He is 21 while you just turned 18, you were considerably better at making kingdom based decisions while he was better at fighting and ruling the army

You father who was sick and didn't have long to live ,made you two fight to death for the role of becoming the King or Queen

Right now ,you and your brother were in the arena, looking at one another in the eyes