The Royals of Sea and Song

Rory, a merman just wants one thing in life: to become a good king and lead his tribe of merfolk who live in the ocean, but when a mysterious illness strikes and his people start dying, he begins to investigate who is behind it and why. When he is spurred by tragedy to go ashore to speak to the human king himself, he makes a deal with Skelmis, the Sea Warlock, to turn him human for seven days. But the price Skelmis demands is far higher than Rory imagines.