Sky on Fire — Lindsay Buroker

The dragons of eld, once benevolent allies to mankind, have been infested by a magical parasite that makes them cruel and aggressive. They've flown through the ancient portal to invade Torvil and plan to kill or enslave all of humanity.

If Jak and his allies can't figure out how to destroy the parasite, a parasite the powerful dragons themselves were helpless to thwart, everyone they care about -- everyone in the world -- will be doomed.