"FABLEHAVEN: KEYS TO THE DEMON PRISON," by Brandon Mull, Shadow Mountain, 594 pages, $21.99 (ages 9-12)

In the summer of 2006, an unassuming little book named "Fablehaven" was published by a somewhat unassuming publisher, Shadow Mountain.

Almost four years later, neither publisher nor book series is unassuming any more. Since then, Shadow Mountain, an imprint of Deseret Book, has gained a reputation for clean books that appeal to mainstream audiences. And the Fablehaven series has earned New York Times bestseller status.

At the helm of the Fablehaven books is talented writer Brandon Mull, who has created an endearing series that will be enjoyed for years to come.

But all good things must come to an end, and the end is indeed nigh for Fablehaven. This Tuesday, the fifth and final book in the series, "Keys to the Demon Prison," will hit bookstores.

Fablehaven is one of five disguised preserves for fairies, satyrs and other magical creatures. Grandma and Grandpa Sorensen are charged with protecting the preserve and are also members of the Knights of the Dawn, a group formed to combat The Society of the Evening Star, which has been causing havoc throughout the magical world.

"Keys to the Demon Prison" picks up about three months after the end of "Secrets of the Dragon Sanctuary."

Kendra, Seth and a group of knights have returned from the dragon sanctuary Wyrmoost with the Translocator, one of five keys needed to open the demon prison, Zzyzx.

The knights now have two of the keys, but the Sphinx, leader of the Society of the Evening Star, seems to always be one step ahead of them. He's after the artifacts, too. Only he wants to use them to open the demon prison, not keep it closed.

Opening the prison would surely lead to the destruction of not only our world, but also that of the fairies.

Now, Kendra, Seth and the Knights of Dawn must race around the world, and, with any luck, beat the Sphinx at his own game while recovering the artifacts.

As with "Dragon Sanctuary," most of the story this time around takes place outside of Fablehaven — though some of the preserve's colorful characters make an appearance and some key action does occur there. Again, this is a smart choice by Mull. His universe has become too expansive for one location.

And the expansiveness isn't just limited to the scope of the story. Mull's characters have developed way beyond what one thought possible at the beginning of the series. Seth in particular is no longer a whining, bickering boy. This once hyper-annoying youth still has a wild streak, but it is tempered by his growth in age and maturity.

"Demon Prison" is a dense book. And while it may be shorter than some of the later Harry Potter books, it does not read as fast. That's not to say it's slow-paced or not interesting, just that there's so much going on and so many characters, it takes a little longer to get through.

In fact, if there's one thing to complain about here, it is that there's no recap of previous stories. Even a one-page summary in the form of an introduction would be helpful. "Demon Prison" does not stand alone, and fans of the series will almost certainly want to read the previous books prior to this one so events are fresh in their minds.

As with the previous Fablehaven books, there are fantasy elements throughout "Demon Prison," including magical spells, sword fighting, dragons and evil creatures. And death and destruction are ever-present forces.

Mull brings this series to a close with an exciting series of events and more than a few surprises along the way. It's a strong conclusion to an impressive series.

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