The Final Enemy is chapter 49 of Tsubasa Reservoir Chronicles.
Oruha-san has appeared with a demon.
She says that she did not want to appear like this, but it could not be helped. She also tells Seishirou that when Syaoran and the rest of the group came to Clover to collect information, she heard them talking about him, the kiji in the form of a man.
Not everything she told them was a lie. The part of a beautiful man controlling demons was true, but the only difference was that the man was not a demon. In order to make to virtual reality game interesting, giving out false information is one of the main ingredients.
In Fairy Park, the virtual reality system's main purpose is to allow a living person role play in the form of a "player". In the early version of the game, there were no playable characters, there were just interactable characters. Therefore; the people who were possessed by demons did not have a presence because the were fixed characters.
Oruha has a presence because she is a real life person role playing as a demon. Her real life name is Edonisu. All she does is sing in Clover, but at the same time she collects data on demon hunters. This allowed her to become an I-1 level demon.
Seshirou wanted to find her so badly because he wants her to grant him eternal life, or she has something that can grant it to him. Seishirou claims that Oruha-san's real name is Subaru, thinking that she is one of the vampire twins that can grant eternal life. Oruha created the Fairy Park system, therefore; whoever defeats her will be granted invincibility in the game, but not in reality.
Seishirou is upset that the game's power has nothing to do with the vampire legend. Sakura's feather allows the game world to become a reality. Although he cannot control it, if the feather's influence disappears, then the reality it created should disappear as well. As long as the vampire twins are not there, Seishirou has no reason to stay either.
Syaoran begs him to give him the feather before he leaves. Seishirou refuses and asks him if he wants to fight him for it. Syaoran has not mastered his sword techniques yet, but if he does not pull his sword out, he will not have a chance of retrieving the feather. He is risking everything just for the sake of his most precious person. He dashes towards Seishirou and swings his sword, it releases a powerful wall of fire, but he dodges it.
He says that Hien suits him. He also says that if Syaoran and the others are searching for the feather, they are sure to meet again. He then escapes with a magic circle. Mokona ends up being pulled by the energy of the feather, and readies a traveling portal. Everyone is shocked by the sudden urge to leave. Syaoran tells Ryu-ou his real name, and tells him that they are not sure when they will meet again. Then they both vow to each other to become stronger until the time they see each other once more.