Gensou Tekina Kako: Reincarnated in a Magical Past

I was reincarnated in the past and now everything is a world of magic!
After dying in an accident during a school trip, a 16-year-old student appears in a Goddess's palace and is tasked with an important mission that has been pending for ten thousand years in the past: to save the world from a disastrous war that wiped out countless lives. Now Lucius Quaramis will make his way to an ancient, unknown place... A fantasy world dominated by magic and swords! How could such a thing have existed in ancient times? A great adventure awaits the protagonist who is determined to enjoy his second chance at life.

Context
The protagonist travels to an ancient world where magic existed and finds himself subjected to the ambitions of others. His goal now is to play the role of the Reincarnated Hero and enjoy his new chance at life: so he will strive to fulfill his duty, unite all the kingdoms to his cause and discover new adventures with his peculiar power... one that is not his own. It seems that the Goddess who entrusted him with this important mission did not tell him the whole truth.