Syaoran is facing off against Fei-Wang Reed when it becomes apparent that his clone is being used by the magician to power a spell to save Yuko, the Time-Space Witch. Before she opened a shop in Tokyo that everyone came to, she was loved by Fei-Wang, and when she died, he was desperate to save her, so he cast a spell to hold her frozen in time. While it was in place, she was able to operate the shop and help those who needed her, like Syaoran and Sakura and their companions.
But with the end of the spell, she at long last faces true dissolution and death, and Fei-Wang, having truly loved her, is desperate to try and get her back. So he reveals that he has not only cloned Syaoran, but Sakura as well, and he is using their magic to try and find her again within the multiverse and bring her back to be his love. But one of the reasons he has for doing so is to prove himself the greatest magician in the multiverse, better than Clow Reed, and because of that, it seems that Yuko no longer loves him, if she ever did- because he is not the man he was.
When she disappears, presumably for good, he seems to go a little insane, but Syaoran and Sakura fight him at that moment, and Yuko appears to their clones, telling them that because they are clones, they can come back from death and choose to be reborn on another world. They do so, and grow up apart, Sakura in Japan and Syaoran in Hong Kong, so that when Sakura's school travels to Hong Kong on a class trip, they meet again and are allowed to marry and live out their love. Sakura becomes pregnant, and their child, their son, becomes the real Syaoran, the one who travels to the Kingdom of the Clow and meets the real Sakura, falling in love with her and giving up his life on earth to try and prevent her from losing her soul in the first place.
But his absence must be made up somehow, and Watanuki Kimihiro takes Syaoran's place on earth, and eventually meets Yuko, becoming her servant to try and take away his ability to see spirits. When she is freed from the spell that held her after death, he also takes her place as the owner of the shop, and the souls of the Clone Syaoran and Sakura are put into an item in the shop, becoming that which powers the shop. But when Fei Wang finds them, he makes them fade away, leaving behind two feathers. This draws the real Syaoran and Watanuki into the space in time where they were being held by Fei Wang, and both must sacrifice something to escape. Watanuki gives up his ability to leave the shop and Syaoran must give up something similar.
But will Syaoran be forever alone, and will he and Sakura ever be free to just live out their lives together in peace and happiness? And what will become of Fei-Wang and his machinations? Will Yuko ever return to see Watanuki again? And what did the manga all mean?
Wow. This manga was one of the densest and twistiest of all the manga I have ever read. Its plot was incredibly convoluted, even by CLAMP standards, and the ending volume, this one, was long and sometimes hard to parse. So much was going on, and the fighting scenes had so many wavy lines that it was more than occasionally hard to figure out what was happening, and what the panels meant. It's also a rather recursive manga, in that the story wraps around to near the beginning in a way, but the universe it is happening in is changed due to the actions of the main characters.
Even the ending of the manga insinuates that more is or will be happening, as the ending words are "And the Adventure Continues...", which is fine for a recursive manga, but it makes the reader have to determine what is the true happy ending for him or herself. The ending of XXXHolic implies that the sacrifices that both Watanuki and Syaoran made were not permanent, but the timeline of that ending makes it still seem like something of a tragedy.
This is the ending of the series, and it completely fulfills everything readers may have wanted in an ending, with a lot more that might not make them very happy. But it's a series that takes a long time to parse and understand, and the ending will be something that readers will chew on for days, if not longer. This is also a very large, long volume, almost 300 pages, and the story serves up both ending and mindscrews enough to make you scream, while also making you want to scream for more yet. Recommended, although not a very easy read.