Return of the Dragon Slayers - Dragonwatch Book 5 - Brandon Mull

Dragonwatch book 5, book 10 of Fablehaven altogether. The magical world teeters on the brink of collapse. The Dragon King, Celebrant, has united the dragons into a vengeful army, and only a final artifact stands in the way of them unleashing their fury against humankind. With established allegiances shifting under the strain, Seth and Kendra find themselves in desperate need of new allies.