Aquaria's Wrath

Aquaria is a peaceful place and home to a race that we would call mermaids and mermen. A few brave Aquarians ask to live on land and create a new subspecies. The Aquarian queen, Marshra, grants their request under the condition that, should she decide the species is no longer worth keeping (they become too dangerous and harmful to things around them or something similar), then she can drag them all back to the sea to return to their original forms, thereby ending the land-dwelling subspecies of Aquarians. This was the beginning of mankind. Many centuries upon centuries later, Marshra's back to check on them and she doesn't like what she sees.