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In the kingdom of Albyn, men who can shapeshift into dragons rule the land. Princess Kianna, the witty, ambitious, sometimes socially awkward heir to the throne, has always wished that she too could be a dragon but, since women lack the ability, she must marry one instead. 
During the final banquet of a festival, Kianna’s beloved father is poisoned. He falls into a temporary coma and, despite her grief and worry, Kianna is forced to begin searching for a husband sooner than she planned.  She enters into a careful courtship with Alun, a dragon from a family of scholars, whose love of books and humor intrigue her.
When Kianna’s father wakes, he lacks his previous strength. He encourages her to agree  to marry Alun and he  soon proposes to Kianna in the castle’s library. Even though she  never expected to fall for him, Alun’s expressed desire for her to rule as his equal ignites a growing affection within her.
As the soon-to-be Queen, Kianna is assigned two siblings as bodyguards ; one, a secret sorceress, befriends Kianna and offers to teach her forbidden magic, while the other, a stubbornly loyal dragon who doesn’t just guard but lurks, makes Kianna question her feelings for Alun. With this odd pair in tow, Kianna embarks on a visit to Alun’s family estate to get to know his relatives.
While there, Kianna spends time with Alun’s younger sister, intimidating older brother and his brother’s equally vicious wife, Raena. One night, the two older couples  spontaneously decide to go out for a clandestine flight and come upon the younger sister’s body washed up on shore. It appears to be an accident but at the sister’s funeral pyre, Kianna uses her budding magical abilities to determine that the sister’s life force has been drained from her body. Kianna knows that only someone well-versed in dark magic could do such a thing, and soon she starts seeing disturbing connections between Alun’s family and a dangerous cult that has been linked with disappearances across the kingdom.
Worried for Kianna’s safety, Alun insists that she return home immediately. Kianna and her sorceress bodyguard set out with a small group of dragons but when they stop for the night at an inn it catches fire and Kianna is kidnapped  and left to die in the remote canyons. She fights for a way out, pushing through the dangers of exposure and using magic against the even more dangerous creatures that live there. Just when Kianna has won herself a brief reprieve in battle with a monster, her second bodyguard swoops in and flies her safely back to the castle.
They arrive to the news that Kianna’s kidnapping was planned to lure her father into danger and he was killed while searching for her. Alun’s older brother, who was present when the king died, gives a testimony that casts the family of Kianna’s bodyguards as the main suspects. After her father’s funeral, Kianna is forced to preside over a sham trial that convicts her closest allies  of treason and sends them into exile. Before they leave, she secretly meets with her bodyguards to arrange magical means to keep in contact. Kianna’s grieving mother moves up the date of her wedding to Alun, and Kianna must swallow her pain to step up as the leader of her country.
Alun and Kianna marry and are crowned in the same day. When they retire for their first night as a married couple, Alun slips a drug into Kianna’s drink. She wakes up to find not Alun, but his sister-in-law Raena, waiting for her. Raena explains that she is a powerful priestess serving one of the old gods of Albyn who has been imprisoned for a millenia. Alun was supposed to kill Kianna, not drug her, but he bargained for her life and now Kianna has a choice . She is taken to a secret lair where the old god offers her a deal: he will grant her deepest desire, to be a dragon, in return for control of the kingdom so he can re-establish a tyrannical reign over Albyn’s people.
Though the word, “yes”,  longs to trip from her tongue, Kianna refuses the offer and the priestess  attacks her, trying to spill her blood as the last necessary sacrifice to free the old god. Kianna is injured but combats the priestess with magic, escaping as the old god bursts free. Kianna’s bodyguards are waiting for her , having tracked her location, and they ferry her out. Kianna looks back just in time to see the priestess  emerge, transformed into the first female dragon.
The priestess, the old god, and Alun set up a false government in Albyn, with Kianna’s mother as their prisoner. Meanwhile, Kianna takes refuge in a neighboring country where she can build new alliances and raise a rebel army. Alun sends Kianna a message begging her to forgive him and come back to be his queen, but his betrayal destroyed any love or trust she had with him. She responds with a declaration of war, and vows to take her kingdom back by force.
: Why must Kianna marry a dragon? Is this ordained by her country’s tradition, her parents’ wishes, or is this something she wants for herself so she can get as close as possible to being a dragon?
: Why does her father’s coma force her to search for a husband? Is this a partiarchal society where women can’t rule in their own right?
: Delete ‘to agree’
: Substitute ‘Alun’ for ‘he’
: Substitute ‘Kiana’ for ‘she’
: This reads a little confusing: at first, I thought you meant that two of Kianna’s siblings were assigned as her bodyguards. Instead, unless it’s significant that they’re siblings, just say ‘two bodyguards’
: Who are the two older couples? If it’s Alun, Kianna, and Alun’s brother and sister-in-law, I’d just say that
: Kidnapped by who? For what purpose?
: Who are her closest allies?
: In the next sentence, you introduce the choice, so you don’t need to refer to it in this sentence also
: Delete the commas around the word ‘yes’
: Since you’ve already named Raena, use her name here, too
: I thought they were exiled?
Overall, this is a very strong synopsis. I was able to follow the main plot throughout, and it’s clear how Kianna changes over the course of the story. This is a rare instance where you may want to name a few more characters – maybe Kianna’s two bodyguards, since they appear to be very important characters and they’re present throughout the synopsis. Most of my comments were small questions and quibbles; you’re very close. Best of luck with this!