The midnight realm

"When ocean animals start disappearing from the reef and end up in the mysterious midnight realm--Naya and her mer-friends decide to find out why. The group sets out to investigate, and when they reach the midnight realm, they find a glowing cave inhabited by a tentacled monster. Then the monster traps Luna, and Naya and her friends need to use all of their creativity to pull off a rescue mission"--Provided by publisher.