Long ago, mermaids traveled freely throughout the ocean. Their kingdom gained several trading partners among the land dwellers. They braved The Deep whenever they wanted, and they built their livelihood in the trench. Their people were both feared and respected throughout the ocean and the continent.
Sailors did their best to placate the tribe in the open ocean. They made treaties of non-aggression to allow for safer passage in the waters. Having the mermaids on your side would inevitably be a free ticket to wherever you needed to go. The tribe could easily turn the tides against you if you became their enemy.
Yet, some hundreds of years ago, all of that started to change. A large Mosasuar settled in The Deep.
This creature was not friendly. Lord Mosa consumed any mermaid that crossed his path. However, all mermaids had in order to enter the rest of the sea. With each of their tribe eaten, the mermaids began fearing for their safety.