All right then, my—uh—gibbering and loyal subjects. This here Trident thing means I'm your king now.Jack Sparrow
The King of the merfolk, also ruler of the merfolk, was a title given to whoever wielded the Trident of Poseidon. As king, they would have the power to rule over all mermaids and basically control the entire ocean. In his teenage adventures, Jack Sparrow was given the title after freeing the mermaids from the oppression of Morveren, Aquala and Aquila but he declined, believing that even as king he wouldn't truly be free. Jack bequeathed his rule to Tonra, the purple-tailed merman.