Tsubasa, Reservoir Chronicle, Vol 21 by Clamp

With Fai's return to his homeworld of Seresu, his hidden past is revealed. How he was set as a trap by Clow Reed, and rescued by King Ashura-O, who put his twin brother Fai into stasis, so he would not decay.

Fai cut his hair, which was originally very long, so that a part of him would always rest with his brother, and Ashura-O added a stone of Protection, Fluorite. Yui/Fai took his brother's name, and Ashura-O added Fluorite as his last name.

As he grew, Fai grew to be Seresu's magical protector. Aware that Fai was cursed to kill any magician stronger than he was, Ashura-O Extorted another promise from Fai, that he would kill anyone he found killing the people of Seresu. In exchange, King Ashura-O gifted him with his markings, that would prevent his power from growing too quickly.

But when a beast starts killing the people of Seresu, Fai is ready to go and see if it really is a beast. Ashura-O reveals his greatest secret. *He* is the beast, and he wants Fai to kill him. But Fai cannot, which is why he put Ashura-O to sleep and left Seresu, hopefully forever. Now that he has returned, but when he still cannot kill Ashura-O, the man who for many years served as a father to Fai, he instead demands that Fai kill Kurogane, who has been nothing but kind to Fai.

Can Fai do it? And how will he react to the real Fai finally dying? Can he find it within himself to Kill Ahura-O to save Kurogame? Or will Clow Reed finally get the upper hand?

So we finally get to see what Fai has been hiding all this time, and it's a doozy. Two curses, and a fairly thorough mind-reaming by both Clow/FeiWang Reed and Ashura-O, who raised Fai so that Fai would kill him. It's no wonder that Fai was so emotionally damaged and hides his feelings behind a false smile.

But King Ashura did a little too well as Raising Fai to be loving and well adjusted, and when the time came, Fai couldn't bring himself to kill the man who raised him. But if Fai doesn't kill Ashura-O, how can the curse on him be broken?

One mystery is ended, but another begins, drawing us deeper and deeper into the web. What is really going on? I'm not sure, but I want to find out. Recommended.