This synopsis will contain spoilers!
Seth makes a promise to the demon Graulus that he will use the Sands of Sanctity, if they obtain them, to heal the demon. Seth and Kendra head out soon after to Obsidian Waste, but when they arrive the discover the Society is already there. They make a quick escape, and head to the Obsidian tower that contains this preserves artifact. The group fights their way through the traps, and ultimately find the artifact (the translocator). Seth and a few others are captured, while Tanu and Kendra escape using the translocator (which can transport up to 3 people anywhere they've ever been before).
Seth is taken to Living Mirage, the 5th and unknown secret preserver, where the Sphinx is the caretaker. We learn that the Sphinx has the Sands, the occulus, and the font of Immortality (which is a unicorn horn). If he gets the other two, he will open the demon prison and attempt to control he demons by entering into a pact with them.
In his prison cell, Seth meets Bracken, a unicorn (it is his horn that is the font of immortality) in human form. They become friends.
Using the translocator, Kendra rescues Warren from the magical knapsack. Grandma and Grandpa Sorenson open a secrete chamber in their house that Patton Burgess said was for emergencies only. In it are instructions to use the Chronometer to travel back to his time and discuss what they must do.
Vanessa reveals her final secret - Kendra's other grandparents (Larsen) are still alive, and are undercover Knights working inside the Society. Together, they plan an attack of Living Mirage with the Translocator, to rescue Seth and get the other artifacts. The group is betrayed, however, and Kendra and the others are all captured. Warren escapes, but is stranded on the preserve. Kendra meets Bracken, and they become friends. Seth is given the translocator and Sands of Sanctity by an unknown agent, and he uses it to go back to Fablehaven. He goes to Graulus, and heals him, but is betrayed. Graulus destroys the pact in Fablehaven and goes to Living Mirage where he releases Nagi Luna (a demon the Sphinx was using to work the Occulus).
Coulter is grievously wounded by Graulus' attack, but not before he uses the Chronometer to travel to see Patton and tell him what happens. Patton leaves secret instructions for Seth that tell him to seek out Vasilis (a legendary sword). Patton also tells Seth how to reach the Shoreless Isle (where Zyzyx is) so he can battle the released demons. He recruits his satyr friends, Hugo, and Vanessa to aid him in his quest.
Meanwhile, the Sphinx has been betrayed and so he begins to work against Nagi Luna and Graulus to prevent the demon prison from being opened. He helps Bracken and Kendra escape, and reunites them with Warren. They flee Living Mirage and attempt to protect the 4 eternals (mortals who must be killed before Zyzyx can be opened), but they fail. At the last battle, however, Bracken is able to recover one of his horns, making him much more powerful.
Seth, Hugo, Vanessa, and the satyrs go through much difficulty, but they find Vasilis and meet up with Warren, Kendra, and Bracken to journey to the Shoreless Isle on a ghost ship. When they arrive, they solicit the help of the Fairy Queen, who sends her Astrids (Kendra is able to turn them into great warriors) and Raxtus (the fairy dragon).
The demon prison is open and the groups fight the demons back. The Sphinx recovers the translocator, and Seth uses it to enter the heart of the demon camp. There he kills Graulus and Nagi Luna, and the Sands are recovered. When he returns, Kendra takes Vasilis, and uses it to kill the demon king. Then, the Fairy Queen empties her kingdom, and all the demons are driven into it with the help of Agad and the dragons that arrive from Wyrmroost. They are sealed inside. The Fairy King is discovered alive (though traumatized from years in the demon prison). We learn that Bracken is there son (they are all unicorns).
Agad brings Mendigo back to life, seals away the demons inside their new prison in an even stronger spell. The Sphinx becomes a new eternal, and Bracken and Kendra decide to consider having a relationship. The pact at Fablehaven is reestablished, and all the family goes there to live together (including Kendra's parents who now know about the magical world.)
Keys shares many of the qualities that made the rest of the series great - it is full of imagination, humor, and interesting characters. On the whole, I enjoyed reading Keys …- April 10, 2011