Percy Jackson: The Firstborn

Kronos and Rhea had seven children, not six. With the help of the firstborn, the Gods win the war, but unfortunately, he had to leave his family behind because of Zeus' paranoia. Only to come back one thousand years later, and what he finds is almost insulting. The pantheon that he called home is a mess: gods obsessed with titles, politics, and ego, more focused on power plays than protecting what they claim to own. Family means nothing when thrones are involved, and “justice” is whatever the strongest decides that day. This story takes place in the High School DxD world, long before the anime begins. Canon will take part later Yes, this is harem, but don’t expect the MC to smash everything he sees. He will have fun, though, just not like an idiot. Along the way, the story will also collide with the Naruto era in the search for power and peace. — Description from novelcodex.org