I just found these spoilers.....
It really seemed like there was no way out to get to the ending that was promised in Volume 1. It was obvious that Shuurei would somehow survive and that Ryuuki would win somehow but I really did not see how it would happen so when it did happen it was nice.
One thing I disliked is that almost everyone survives. It is a shoujo light novel so it was obvious that at least Shuurei would survive but people like Anju I couldn't see surviving but somehow did. I did want something from Sakujun more even though he was dead. (He helps Shuurei escape from Anju in volume 18 as a corpse and that's pretty much it) What I did like is that it ended the same way that it began. The storyline got darker and darker and darker throughout 13-17 but 18 brought it back to humor and romance.
Plot line Spoilers:
Volume 17 was all about Ruka, fixing the locust problem and getting everything set up for Ryuuki vs Ouki
Shuurei figures out the 2nd boss, where the money, the steel, was going. it's Anju. The money went to bribing the Kou Family. The steel went to Ouki's property where he got some steel workers from the Red Clan building weapons.
Ruka, Uu, Shuuei, Ouki fix the locust problem using a virus against the locusts and Uu uses the last of his life force to make sure that the locusts die.
Seiran and so forth help Ouki to help Ryuuki. (Killing Ouki would kill Ryuuki politically even more so than letting him take the crown.)
Shuurei sacrifices the little time she has left to help Ruka fix the holy objects. When Ruka is killed by kyonshi Sakujun and the balance of the world goes array. Ruka Uu, the girl who caused Ruka's death, Eiki all willingly turn their spirits into human pillars. Uu and Ruka were 2 halves of a single artifact because of how long Ruka had lived and protected the world.
The Hyou family is completely opened to the world, helping the weak and supporting everyone.
The mirror is fixed by Karin. (Karin's husband is the human vessel for Hekisen). (Creating the mirror did not kill the creator, that was bullshit that someone thought up and all the creators suicided because they would never admit that they weren't good enough to die for their art)
Shuurei goes to rest in a casket. She'll have 1 day more to live and only when she and the person she gives the key to want her to will she awake. If she stays asleep she can live 10 more years in sleep.
Volume 18 was about Ryuuki. Shuurei spent 90% of the novel sleeping while Ryuuki began giving up things to protect them.
Ryuuki learns from Shou Taishi that his responsibility was to last 3 years as King. He was manipulated to make it easier for Ouki to become King
Ryuuki still believes of Yuushin's betrayal but decides to escape the castle before more people on Ouki's side try to kill Yuushin.
Ryuuki gives up one of his swords to a stranger while escaping for helping him because he doesn't need the sword. This is direct contrast to Ouki who gave everything up except for his sword and armor because he still needed them.
Ouki takes control of the government and fixes up a lot of the problems. He uses many of the plans that Ryuuki and Yuushun made but couldn't announce because of the backlash that was going on against Ryuuki.
Ryuuki spends the next few months with sleeping Shuurei, Shuuei, Shouka so forth. Shuurei is protected by Hien, Riou's mother and Ouki's daughter who was the last body for Ruka. Hien was dying in childbirth when Ruka put her into stasis in hopes that someday she would be able to be fixed. With Ruka dying Hien uses the last of her time to work towards helping Shuurei.
At this point Ryuuki's basically ****ed. The 6 ministers are not on his side and are helping Ouki fix the government. The Ran providence locks up the governor as sick and the rest of the government is taken over by Ouki's supporters. The White and Black Providences are angry at the Red Providence who was hiding food when the locust problem occurred. They are also pissed off at Ryuuki who failed to help them. The Yellow family has been accepting bribes from Ouki and in general always sides with the winning side. The Heki Governor is missing from the earthquakes and Heki people in general are not terribly interested in war.
Shuurei is stolen using Sakujun (who was turned into a Kyonshi by Anju who is his older brother) and used against Ryuuki. Ryuuki decides to not try and save her.
Ryuuki and Ouki meet at a battlefield and have a duel. Ryuuki arrived with just Shuuei, Ouki with 50,000 troops.
Shuurei wakes up and rushes to Ryuuki's side with the help of her old friends at the government. Rou Ensei is who she trusted with the 2nd key.
Ryuuki and Ouki duel.
Suddenly a village in Ouki's territory rises in flames (due to Anju's plotting)
Ryuuki offers Ouki his head to save Ouki's troops from death. He can't stop them as a King but his head can.
Yuushin arrives with the support of White/Black/Yellow families. He was gonna betray Ryuuki if he hadn't run away to protect Yuushun's life. Because that would be what a normal guy would do and if Ryuuki was just average then Ouki is better for everyone. However, because Ryuuki follows through with his beliefs and sacrifices everything to protect it, Yuushun sides with Ryuuki and Ouki loses the battle.
When Ryuuki escaped the castle Yuushin asked Haku Raien to give Ryuuki his sword. This sword giving was intended as Haku Raien's personal loyalty to the king but was in reality, the White providence pledging to the king. The Black providence was given laxatives in the guise of medicine that would make them stronger. They pledged loyalty for the cure and are now honor bound. Kouyou somehow convinces the yellow clan but gets lost in the north. The Brown/Red/Blue families also come and Ouki loses
Ryuuki makes sure Ouki doesn't commit suicide and gives Ouki, Bakuya which he accepts and pledges loyalty to Ryuuki. On a side note, his general was a legendary fighter from the Black clan.
Shuurei is shot by an arrow. (this is cliffhanger)
The 7 Sen meet and mourn the death of one of their members and most of them pledge not to meddle with things for at least another 100 years. Everything went to plan and the Black Sen's love still didn't show up, Purple just lost. Yellow doesn't want to have more work saving lives, Heki has a son and wants his son to live a nice life, Sa doesn't really care much, Byakuya completely fell for a human.
Shuurei survives using the life force that was being tapped from her via Kyonshi Sakujun. (her life was being used to reanimate Sakujun) Hien, Ruka, and Shusui were basically sucking back her life force back into Shuurei. She's given just a little more time.
Ryuuki proposes to Shuurei but is rejected publically for the 100th time. (In the background Yuushun says that Shuurei is better as an official for the country and Shuurei's old friends all petition against the king.)
Reishin, Yuushin become parents. (As a side note, Yuushin wasn't dying but pretending to die because it's safer. His wish was to become a good person. He likes siding with idiots because he doesn't have to think much) Yuushin decides to never side with the King when it becomes matters of women and love after being beaten up by his wife who hid her pregnancy from him.
Jin and Shuuei are given the chance to return Jin to Shiba family if they defeat White General who is the current owner of one of the Shiba family's old family heirloom sword that they lost 100 years ago. Juusan hime says she'll consider Jin when he defeats the White General. Juusan Hime privately gives up on Ryuuki whom she was quietly in love with. Jin is made the vice minister of Military division.
The entire Ouki taking over thing is summed up as Ouki getting pissed off at the king and going to pick him up from the Red Providence. Ryuuki makes Riou his son. Ouki laments of Riou's lack of luck in fathers.
Shuurei is given a job like Seiga that allows her to visit the world of Saiunkoku over 3-4 years. Ryuuki allows it and says she'll always come back, everyone will come back. They have a very sweet moment. Shouka overhears things and is reminded of Shuurei's mother and his own romance. It'll take a while to get married.
Ryuuki's reign truly begins in Jyouchi 5. His reign is noted for having no wars and for cleaning up after the 100 years war. Much like his father was compared to Sougen, he is compared to Soushuu. He's also famed for having only 1 wife.
At age 32, he marries Kou Shuurei. At that point she was a famous official but she gives up her official status and accepts Ryuuki's proposal.
This is also the year when a new girl passes the examinations. This new girl is Shuuran from the Brown Providence and also becomes the first Female Prime Minister of Saiunkoku.
Ryuuki's reign is famed for a period when women were exceptionally famous. Hyou Shusui advanced schooling, Kou Yuri conducted a lot of charity and raised the quality of life throughout saiunkoku. Shuuran governed well. Ran Jyuusanhime was famed in the military while Heki Karin was famed in the arts. Sai Rin became the minister of crafts while being the wife of the Prime Minister.
Kou Shuurei is one of the most famous because her life was not too long.. The year after her marriage, Kou Shuurei died in childbirth at age 30. She died saying "my just a little more time ended" Few records of her life remain leading to her legendary status.
Shi Ryuuki immediately made Riou his successor but Riou gave up his succession right to Shuurei's daughter who lives 70 years as a rare Queen. Riou becomes prime minister after Tei Yuushun Kei Yuuri and Ri Kouyu and restarts the tradition of Hyou family in the government.
Shuurei's daughter is one of the rare Queens of Saiunkoku's history (asides from the ancient queens) and was said to closely resemble her mother.
It's nice to figure out was what except I still have no clue who was the human vessel for the Black Sen
Purple - Shi Shou (Who ends up being the big bad in a way)
Red - Bara Hime (Shuurei's mom, she officially dies)
Blue - Ran Ryuuren (Shuuei's Younger brother with the flute)
Yellow - Kou you (Shuurei's doctor)
Brown - Sa Nan (Ensei's training master)
Heki - Ourou Jun (Heki Karin's Husband)
White - Byakuya (To Eigetsu's other personality)
Black - Anju? (Perhaps he didn't even have a human vessel)