Rave Master Vol 14

From the Tokyopop edition

Someone is after Ellie's Etherion. In order to save the merpeople, the gang infiltrates the Demon King's lair but walks right into a cowardly trap! Ellie is kidnapped right before their eyes! Ellie's Etherion is the key to finishing the ultimate evil weapon, "Mermaid Cannon!" Haru is not about to let them get away with that!

Source: Anime News Network Encyclopedia