Promise Stone (約束の石) was an item used to summon the Legendary Mermaid.
According to legend, it is described to be used to summon the Legendary Mermaid when the stone resonates and is tossed in a specific pool at when four certain stars shine brightest.
A beautiful translucent stone, that one can be able to see the cross sections by adjusting the light.
Promise stones appear to bear a weak magical power to them. However when a magic caster pours their power into them, they cause the stone to glow white and create a resonance. This sound while audible in air is amplified when it is placed underwater, calling the Legendary Mermaid to come.
- A fragment of this item was passed down through the ages at a small seaside village in the Re-Estize Kingdom where the legend of the Legendary Mermaid originated.