The Veiled Master

In an ordinary world transformed by the sudden appearance of giant dungeon towers, a teenager becomes trapped inside one. Unable to escape, he disguises himself as a masked, cloaked training NPC and builds a dojo, teaching students through disciplined martial-arts challenges while letting each choose their own mask and fighting style. He remains trapped for ten years, during which some of his students become legends. When the tower finally allows him to leave, he has not aged a day, leaving the outside world to reckon with the mentor who never grew older.