The battle of the werepenguins / Allan Woodrow ; illustrated by Scott Brown.

Enhanced descriptions from Syndetics:

In the series Chris Grabenstein called "hysterically hilarious," the saga about the evil werepenguins of Brugaria comes to a stunning, action-packed conclusion!

When Bolt, Annika, and Blackburn are given another cryptic clue by Omneseus the Seer, they know the time has come to defeat the baddest Werepenguin, "the Stranger," and free the world's penguins from his evil reign. Only a very special tooth can bring the Stranger down, but of course, procuring this precious item is no easy task, and it doesn't help that Bolt can't stop hearing the Stranger in his head, trying to coax him to the dark side. When Bolt stumbles into a weremole burrow and meets Grom, a human boy with a penguin birthmark who's desperate to be bitten, he starts to question whether a werepenguin can be anything but a terrible monster. Or if one born out of love might be the key to penguin salvation. As Bolt's werepenguin strengths grow more powerful, the ultimate war between good and evil looms closer. But no one can guess what's about to go down. And when an unexpected foe returns, the battle of the werepenguins IS ON.