“Legends. Thousands of them have been spun throughout history and thousands more will be spun. Some sing of heroes, others of villains. But one legend, lost to time, sings of a single homesick soul in a world not his own.”

Mark Tyr went to a local convention cosplaying as Shirou Emiya when he is almost killed in an explosion.

He still has no idea how he ended up in Equestria as Avenger.

A thousand years later, he get's stuck in Twilight's head and they have a goal.

Get him a body and get him home.

Avenger is from Fate/Hollow Ataraxia (A sequel to Fate/Stay Night.)