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Holy Maiden YSABEL has only four months and three weeks to live. It’s a race to see which kills her first : her healing ability which takes a day off her life every time she uses it or the Council of Patriarchs who plan to sacrifice her in a ritual to obtain immortality when she turns twenty-five. Ysabel has made her peace with her fate and uses the political influence it brings her as leverage to obtain political rights for women and protect the halfling refugees camping outside her city. Although by “made peace” she means she indulges in recreational drugs and dominatrix games to forget. She also suffers from face-blindness and anxiety attacks. 
Then Dark Lord KAINE arrives half-dead at the doorstep of her clinic , fleeing an assassination attempt by Patriarch JIOH. Ysabel sneaks him out of the city so he can stop his army from attacking her city. Before leaving, Kaine tells her he owes her a great debt which he vows to repay.
The next she hears of him, he’s got an army outside the Holy City Arahasnor and is demanding the patriarchs turn her over to him. The council sells her out like cheese about to expire. Still traumatized by the last clichéd dark lord who tried to carry her off, Ysabel hides a knife in her hair and plans to save herself.
After narrowly avoided being stabbed, Kaine explains he did it to save her life after he discovered that the council and their leader JIOH plan to kill her and have bound her with a magical oath not to escape. It definitely wasn’t a dreadful attempt at kidnapping equal love! Although, he’d be lying if he said he didn’t have any feelings for her…enough to entrust her with his greatest secret, that he’s a trans man who uses Bodymodifiers. 
Ysabel has become accustomed to giving up and sacrificing herself , but Kaine persuades her to let him try to free her from her geas. The plan is to win World Game, a proxy war where nations compete for territory and money using teams of six wizard-warriors. The world leaders are magically bound to abide by the outcomes. Kaine’s gift, Plunder, allows him to steal other’s abilities and makes him a favorite to win. Ysabel has the first team made of women and led by her best friend, Humanity’s Strongest Wizard-Warrior ALZIRA, who can control meteors. However, she’s been too scared to compete ever since the patriarchs threatened to murder all her friends if she tried, and it takes reassurance from them to find the courage she’s been suppressing. 
Ysabel and Kaine’s teams meet in the finals and fight to a draw. When Kaine is injured, Ysabel admits her love for him. Now she’s been freed of her oath and become the new head of the council by defeating all the patriarchs , all seems well. In a vicious twist, Jioh reveals himself to be a former dark lord with necromantic powers who takes over the city. Imprisoning Ysabel’s friends, he forces her to cruelly reject Kaine.
Ysabel’s teammates break out of jail and come to save her. Kaine breaks into the city to save her too despite her earlier attempt to chase him off. He fights Jioh despite his magic not working and is fatally injured while killing the lich . He refuses to let Ysabel heal him because she is already close to the end of her lifespan, despite her pleas. When she tries to heal him, he uses his ability to steal her cursed gift, while she tries to stop him because she doesn’t want him to end up with her curse.
The result of their clash of wills is that Ysabel’s life is restored but Kaine loses his ability to steal magic. Ysabel continues to heal people , but from Kaine, she’s gained the power to take a day of the patient’s life instead of her own. She marries Kaine and moves back to his kingdom. In the very last scene, she overhears two kitchen maids telling the story of how the dark lord abducted the holy maiden. They’re both under the illusion that Kaine is the dominant one in the relationship, although actually he’s submissive by nature, as is revealed when Ysabel heads back to her room. But she no longer cares what others think of her and is content with her new life.
I think this is better phrased as ‘which will kill her first’
This last sentence seems tacked on, and unnecessary to this synopsis, since neither her face blindness nor anxiety attacks recur later. I’d delete it.
This is the first mention of ‘her clinic.’ I assume it’s where she practices her healing, but I’d make that clear.
I’m very confused by this sentence. Is the ‘he’ you refer to Kaine? Why was his army about to attack the city? Did he have a change of heart about attacking, or is not attacking the favor he plans to use to repay Ysabel?
This phrase doesn’t fit the tone of the rest of the synopsis.
I don’t understand this phrase (kidnapping equal love). Did you leave out a word or punctuation?
Why does Kaine entrust her with this secret? Is it a secret because he’d be killed or deposed if it got out? The fact that he’s a trans man doesn’t seem relevant to the rest of the synopsis, so I’d either cut it or refer to it again later, assuming it’s important to the plot.
But earlier, she had a knife in her hair so she could defend herself…
What are ‘geas’?
This would be better phrased as, ‘which is led by her best friend…’
Is the ‘she’ Ysabel or Alzira?
This is cool, and I like the idea of a wizard game that leaders are magically bound by. It’s different. But I’m not quite sure how it ties into the stakes of the book (which I think are Ysabel being freed from the council? I’m not totally sure). Take some time and see if you can distill the stakes into a single sentence. Once you have, see if you can better explain how the World Game relates to those stakes.
Isn’t it Kaine’s goal to get Ysabel freed? Why would they have entered separate teams? Wouldn’t they have collaborated on forming a single super-team?
When did she do this? How?
But he was already a bad guy, right? Why is this a twist?
What is this?
Is she still head of the Council? I thought the plot was going to result in a more political role for Ysabel, but maybe not?
To make this stronger, really focus on identifying the stakes in the book. Why must Ysabel take action? If she doesn’t act, then what? Once you’ve done this, make sure every step of the main plot described in the synopsis relates to those stakes and works directly for or against Ysabel’s journey. Right now, the main plot is a little muddled because I can’t quite figure out how the World Game fits in with Ysabel’s stated goal (I think) to become free.
Overall, it’s an interesting story and you just need to work on clarifying some points in the synopsis. Best of luck with this!