Heaviness of infinite time (bxb)

In this world, there are two types of wizards, light magic wizards, and black magic wizards. Your type of magic depends on what demon you have. Black magic wizards are very rare because to be a black wizard have to have a high-level dark demon. A high-leveled demon has very powerful magic, it would destroy the wizard's body if the wizard couldn't handle it. what happens when a boy, whose father was a black magic wizard, has a very powerful soul, that could handle all demon kings together? what happens when the demon kings take interest in him? will they mark him? will he accept to be their wizard?