Different in Blood, same in spirit

Y/N is a human, living in hell with his adoptive parent, who later on has another biological son named Stolas. They both grow up, live together but have complete different paths. As Stolas was chosen to be the Goetia who studies the cosmos and the heir of the family, Y/N path was still unknown. Why? He was still a human after all. So he chose the complete opposite of Stolas' path, becoming more a man of fighting and combat. Years pass and they both learned more and more. Stolas teaching Y/N about magic and the cosmos, while he teaches him about survival, fighting and how to minimize damage when shot, stabbed or punched/kicked. Of course, since it was hell, some people hated the royal family, others who hated humans. Coming together they tried to get them both killed, which would have succeeded if it wasn't for Y/N and his combat skills. Protecting Stolas and himself, he needed to chose between the two promises he made... he broke one to keep the other one alive.