The Isle of Eternal Happiness — Kay Berrisford

After several months of wedded life, Ben is feeling the pressure. Not only has he a job and marriage to worry about, he has to sustain Lyle's waning magical powers and see through the destiny of the Dragon Rider and save Lyle's merfolk family in their hour of need. Lyle's concerned that Ben is changing for the worse, acting secretively and strangely. With Ben rapidly developing powers of his own, Lyle suspects Ben is being corrupted by the sinister legacy of Prince Clewell, whose magic sword Ben has inherited. If not, their marriage is failing. Ben tries to pacify Lyle by whisking him off on a belated honeymoon. But Ben's true purpose is to seek a hidden fairy sanctuary, "The Isle of Eternal Happiness," and to fulfil the Dragon Rider prophecy. He's even invited Lyle's dragon-shifting sister, Cully, to help him in Lyle's stead. Lyle finally snaps. Ben's mission might be a noble one, but magic is turning him into a monster. And Lyle is still a force to be reckoned, even with his health and powers deteriorating.