Guardian's Vow

Tane and the rest of the island crew have gotten Mercury Island back and are parked a mile in the air above the islands of Atlantis. They are frantically preparing to meet the attack of the warship Fellstorm, which is coming to capture Mercury Island.
 
When they are joined by Gladiolus Lark, a half-blind girl with a magical malady, the team dynamics begin to shift. Friends draw closer, and even rivals are persuaded to work together. New powers are unlocked as the team delve into the mysteries of Atlanticite crystal, and even the island, itself.

Now the arms dealers National Weapons Enterprises approach to capture the island, and the sea monster Tyrannith waits in the ocean below. Tane must make a terrible choice--to save his friends from one enemy, he must appease the other. He can't talk NWE down ... and appeasing Tyrannith may cost him his life.