A mental battle against the strongest evil spirit just five seconds after being born... and while he was doing that, his magic power ended up getting maxed out!?

Shirakami Kai was reincarnated into an alternate Japan where monsters known as "Spirit Fiends" exist. But when he was born, for some reason, he was caught up in a ritual to summon the strongest evil spirit. A curse tried to kill his mother, after which the evil spirit went on a rampage, trying to take over Kai's body. And while he continued to battle against the strongest evil spirit inside his head, he became an incredibly powerful entity without even realizing it!? + More