Level Up-agi Sileun Cheonmanim

A Goryeo-era prince skilled in martial arts, Jin Yuseong, enters a magical palace gate seeking escape from grief after losing his comrades. He emerges in modern Seoul and becomes involved in a mission to secure a gate breach, attracting the attention of the Safe Gate organization, which safeguards people from such portals. To avoid being recruited by SG, Yuseong assumes the identity of a high school student and the nephew of a former ally, though his past as a warrior may make remaining hidden difficult.