Dainsleif was once the leader of the Royal Guard in Khaenriah. Despite his efforts, he couldn't stop the city from being destroyed 500 years ago. As punishment, he was given eternal life and has been traveling Teyvat ever since.
He is the former knight captain of the Royal Guard of Khaenriah. However, he failed to prevent the destruction of Khaenriah 500 years ago. A curse of immortality was placed upon him so that he would forever wander Teyvat.