Arata: The Legend, Vol. 1 by Yuu Watase
In a mythical world where humans and gods co-exist, Arata has the unfortunate honor of being the successor to the matriarchal Hime Clan. This means he has to pretend to be a girl or face death if his true gender is discovered! On the day of his inauguration, an assassination plot forces him to flee-and he somehow winds up in modern-day Japan! Meanwhile, a boy named Arata Hinohara gets sucked into the first Arata's world and finds himself dealing with people after his life!