Time is now
Those lucky enough to be born with the innate ability to control a specific type of magic are called sorcerers.
Only they themselves can know what is the best path for them to follow, requiring a long time and an enduring heart to make any notable achievements in their lifetimes.
Their magical power often overflows and is not easy to bring under control. Their spells are limited in quantity but can be easily adapted for different situations, giving them an upper hand under most circumstances.
In the case of mermen, the difference in their physical forms is quite an obvious indicator of their innate abilities.
Most normal mermen have superhuman strength, the ability to partially shapeshift when in water, and an intelligence on par with teenaged humans. But some of them are born with magical powers and are in turn superior to others of their race.