Every 20 years a member of the royal family must return to Aquellia and sit atop the ancient throne. When they speak the ancient rite, they gain the power to make Merillia, the life force of the ocean.
The Ancient Rite
"With the Changing of the Tides,
Merillia power will arise.
The royal mermaid on the throne,
Her fullest mer-self now is known."