Inside the Demonkins' lair, Tatsuya and the others reached a room with four mirrors in the middle. When Tatsuya looked at one of the mirrors, his reflection became a dark version of himself and then attacks the team. After the dark Tatsuya was defeated, the Gemstone of Darkness appeared. When Tatsuya accepted the Gemstones, his Traveler's Sword transformed into the Demon's Bane.
In the end, Tatsuya presented the Demon's Bane to the Goddess of Destiny but was not able to get his wish granted because of him being a False Traveler.