The Mirror of the Greatest Love is Chapter 21 of Tsubasa: Reservoir Chronicles.
The townspeople have Syaoran surrounded. The Ryanban states that if he attacks them, Sakura and Chu'nyan will feel the townspeople's attacks many times over. Syaoran attempts to escape, but he cannot risk hurting the two girls. He is overwhelmed by their attacks and lets himself be hit with their weapons. Sakura cannot bear watching him be hurt, so she calls out his name.
Syaoran hears her voice, and realizes that Sakura and Chu'nyan are merely illusions, yet more trickery from the Ryanban. He knows the Sakura he is travelling with would not just call him "Syaoran" (i.e. without an honorific). He lunges toward the Ryanban, aiming to reclaim the feather for Sakura. A townsman steps in just in time, and all of the hypnotized men attack Syaoran yet again.
At the back of the chamber, a young girl steps in with a mirror. It is the real Chu'nyan! She brought a magic mirror owned by her mother, an item that cancels out any magic cast on people. The townsmen confusedly wonder what's going on, and why they're at the Ryanban's castle. Chu'nyan states that she may not be as powerful as her mother, but she is strong enough to use the mirror and strong enough to make sure the Ryanban doesn't use the townspeople for any more of his dastardly deeds.
Two figures show up -- Fai and Kurogane have finally made it up to the top level. Syaoran barely notices all of the commotion going on around him. All that he wants is to retrieve the feather. The Ryanban pleads with the boy, saying that he may be able to bring back Chu'nyan's mother with the power of the feather. If Syaoran takes back the feather, that'll never happen.
Upon hearing his cowardly promises, Chu'nyan explodes. After killing her mother, who was only trying to protect the village from his evil, he has the audacity to make such promises!? She does not believe his words; she knows from her mother that the dead cannot be brought back to life. The Ryanban's promises mean nothing, as she'll never be able to see her mother again.
Syaoran asks her if she wants to take her revenge. He is giving her the opportunity to take the Ryanban's life, but he asks if he is worth it. She says he is not.
Syaoran strides forward, and as he's looming over the Ryanban, long razor-like fingertips reach from behind him...