The Wyrm King
The last book of the Spiderwick Saga, the story starts with the Vargas family going to a family therapist because of the children's latest strange behaviour, which Nick's father thinks is caused by his re-marrying. Meanwhile, sinkholes have been appearing everywhere throughout Florida, where small black serpent-like creatures are vaguely seen by Nick. Under the assumption that these sinkholes are caused by the creatures that the giants were supposed to destroy, the Vargas kids (plus Sandspur) meet up with Jared, Simon and Mallory at Noseeum Jack's house, which is now deserted and has a massive sinkhole in the front yard. After breaking into the house, they find more of Jack's annotations, which compare the hydra, shown as a group of intertwined wyrm dragons, to the legend of the Rat King.
Finally going to Jack's backyard, Nick and Laurie find what Jack had meant to give them: a small pool filled with lime and muck with Taola and two of her sisters, Ooki and Ibi, whom Jack had captured some days earlier. Meanwhile, Mallory goes into the sinkhole to investigate, but finds herself unable to breathe and surrounded by the small snake-like dragons. They manage to get her out and put three of the creatures in a jar. They figure that the creatures produce methane, which makes the air around them unbreathable, and realize that these were the creatures that the giants ate in order to breathe fire (which they originally believed to be salamanders).
Seeing how fast the dragons can grow after feeding them a chunk of pizza, the children all drive to the beach in order to get the giants back to land so they can kill the dragons. Although reluctant at first, the three nixies agree to use their singing to lure the giants, and one of them is taken to the water with Nick and Jared in a surfboard (because since nixies are used to freshwater, salt water hurts their skin). Before they can accomplish this, however, Nick is taken underwater by the merfolk, who tell him how they're glad that the land-dwellers are doomed and give him a magical cap in the process that allows him to breathe underwater. The nixies manage to get some of the giants to follow them to land, where they immediately start eating the small reptiles and fighting each other again. As the children drive away from the giants, their car falls into a sinkhole and only after some time do they manage to get out. A giant then attacks them, being saved by the showing up of Jack Junior, who suddenly decided to listen to his father and come help the children kill the giants (being surprised when they reveal him that killing giants is no longer their plan).
After dropping the nixies in a nearby pond (their reward for having sang to the giants), Jules, Laurie and Nick pass by Cindy's house and notice a large sinkhole close to it. When they go to warn Cindy and her parents, Laurie and Sandspur fall into the growing sinkhole and a tree falls on Jules's legs when attempting to rescue her. When Nick tries to help Laurie get out of the sinkhole, he finds that she still hadn't died from the methane exhaled by the dragons because Sandspur (who had grown to a huge size, revealing himself to be a spriggan) had been eating them, proceeding to eat Nick whole and only regurgitating him when Laurie drops rocks into the creature's mouth to make him sick. Nick and Laurie get out of the sinkhole and go back to meet Jack Jr. and the Grace kids, leaving Jules at Cindy's house waiting for an ambulance (being followed back by Sandspur, back in his small size, like nothing had happened).
Noticing that all sinkholes are connected, they come to the conclusion that the biggest hydra must be in the center, laying the eggs that hatch into the smaller hydras who wander through the tunnels. They notice that the giants are all going to meet that alpha hydra, and Jack Junior leaves to go kill it, followed by the children. There, they meet the giant multi-headed dragon, who had already started to kill the giants that had gathered to fight it. After hurting Jack Junior and the Grace kids, Nick has an idea and realizes it's up to him to save the day. Using the cap given to him by the merfolk so he could safely breathe, he goes into one of the tunnels with Jack's machete so to meet the hydra from below. When he gets there, instead of trying to kill the beast, he cuts the substance that keeps all the dragons together, thus making them separate from each other and easier for the giants to kill.
At the hospital, all the children get their injuries treated, Jules gets back with Cindy, Jack calls his father to tell him what happened, the Grace kids meet their father there and the Vargas family decides to all stay together. The Spiderwick Saga ends with the Vargas going home, being watched by a silent giant in the distance.
Note: The authors are hoping to continue the first five and stop the ending three.
Read more about this topic: The Spiderwick Chronicles, Beyond The Spiderwick Chronicles