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Long ago, the nourishing sun vanished and plunged the planet of Serim into darkness. The stars emerged as heroes, replacing the sun in the perpetually dark sky. But in becoming saviors, they also transformed into oppressors , forcing the monster inhabitants into slavery. Now, whisperings of a secret prophecy give monsters hope they can one day overthrow their tyrants. 
Seventeen-year-old Princess RYE is desperate to escape her life. Half-monster, half-star, Rye is the first of her kind and an outcast among the stars. When Rye was just three-years-old , her monster mother murdered her star father—who happened to be a beloved king—tainting Rye with her legacy. In an attempt to fit in, Rye denies her monster heritage, determined to prove she’s a worthy successor of her father. 
When a star falls from the sky, the council discovers a treacherous plot to snuff out the stars. They task Rye with marrying a prominent monster, ARLIN, under the guise of bringing stars and monsters together. In truth, they want her to find the monster behind the murders and to ultimately turn the stars against the monsters. Although a bit trepid , Rye thinks this might be her chance to prove herself as a star.
Arlin and Rye have a rocky beginning. The monster has a temper, is gruff, and speaks his mind. Even worse, Arlin sneaks the princess down to his monster town, where she witnesses a brutal whipping, revealing the true horrors that plague monster lives. 
As their relationship and feelings grow, Arlin and Rye learn to respect each other. The princess agrees to work with Arlin to bring monsters and stars together on the day of her coronation ceremony, in hopes they can create a peace treaty. They work in secret, knowing the council would never approve.
The day of Rye’s ceremony arrives and Arlin goes missing, threatening their plans. Chaos reigns as stars and monsters clash, but Rye is powerless to stop it. Before she can search for Arlin, the council arrests her for murder of the snuffed stars. They framed her and Arlin to keep Rye from ascending to queen. 
Rye is devastated. But allies rescue her and help the princess discover two things: first is an ancient prophecy about a half-monster, half-star who will rise as a new sun and free the monsters; second, her father, KING STRATTON, lives and has kidnapped Arlin to lure Rye into a trap. 
Long ago, when Stratton learned of the prophecy, he killed Rye’s mother, faked his own death, and spent years collecting the necessary power to steal Rye’s magic so he could rise as the sun. It seems Stratton might actually defeat Rye, but he makes one fatal mistake: Stratton believed the sun could only rise once accepted by both monsters and stars. Really, Rye needed to learn how to accept both parts of herself, which Arlin helped her to do. Armed with this knowledge, Rye’s sun powers emerge and she defeats Stratton.
With the threat disbanded, Rye and Arlin can work together to rebuild a peaceful world in which both stars and monsters can co-exist.
: What was the stars’ motivations for oppressing the monsters? If they went out of their way to save the planet, why would they then want to enslave the inhabitants?
: But if they did overthrow the stars, would they be plunged back into darkness again? Seems like a lose-lose situation.
: Should be ‘three years old’ here
: It’s perhaps worth including a few more details here about how Rye feels like an outcast, what the other stars do to her, etc.
: What council?
: Has there been more than one murder?
: I don’t think this is a word. Do you mean trepidatious?
: How does one become a prominent monster, worthy of marrying a princess, if they’re all enslaved?
: How does Rye discover she was framed?
: Who are the allies? Stars or monsters?
: Is he also the one behind the murders?
: If all he needed to do was steal Rye’s powers, why did he go to the trouble of killing the queen and faking his own death? We don’t need all the plot details here, but if there’s a simple way to explain this (did Rye’s powers only manifest in grief? Was the queen onto his plan?) that would help clarify.
This is an extremely strong synopsis, and my only comments are nit-picks. It’s very well written, and I can follow the main plot from beginning to end. Think about incorporating a few additional details, as I noted, and you should be good to go!