Sea of the Forsaken
Since the beginning of time, mermen and mermaids alike were known to be of existence just as much as humans were on land. However, the great war broke out between the human and merfolk races because of tensions from mermaids luring sailors to their deaths and fear overall that either race would try to gain control for both land and sea. The merfolk had to retreat from their loss, tried to survive by moving deeper in the oceans, and lived under a monarchy of five great rulers that yielded mystic powers that were "granted" by God himself to set order in the sea realm. However, one of the elder rulers could not control her power as her priests have warned her of what would happen if she were not able to control her power and became the "Magna Dracona Aquia" (Great Water Dragon). Her form caused great devastation in the kingdom and was killed in her dragon form by her brother the Prince of Great Strength whom was able to slay her. After the elder princess was slain, the merfolk rioted and went after the royal siblings as well as their offspring to be executed for letting chaos happen in their kingdom. After the royal family cease to exist, the realm was lost in darkness and majority of the merfolk pursued in living on terra land where they would intermix with the humans while concealing their secrets of being a part of the merfolk. Later on, they would have children with the humans to lessen the chance of having dominant merfolk traits so that they would not be executed. As centuries past, the merfolk became more cautious on enduring the family line on land, and the humans later on forgot the tales of that brutal past or saying that they never did exist at all according to their accounts.
So a new story begins with three girls after nearly a millennium had past from these events, and would later be involved into ongoing business in which the merfolk would rise again but whether the path would determine that it be helpful or harmful for both merfolk and humans.