Mer-people:
The half...
...Ocotopusses. Sea-witches, known for dancing on the edges of black magic. Gifted and powerful, a sea-witch can achieve a lot through her magic, though is shunned by most others. (Again, looking at you Ursula).
...fishes. Or standard mermaids and mermen. They are the most numerous and thus are the ethnic majority. Most use their magic to influence the sea creatures around them. Often with special objects like a flute, horn or bell.
...lobsters. Or Cerceans. They have an armored tail and back, with powerful muscles. Soldiers and warriors, bred for war and battle. During peace time they serve as guards. Though they aren't really gifted in the magic of the seas.

I'm just thinking, what about making the relic to control the Leviathan not carry-able? Like to control the Ancients you have to play a specific part of a song on an special organ in Atlantis. So even when you reach the Organ, it's sort of useless until you know what part of what song you must play.

I could help with the worldbuilding, if you like.