In the early 1950s an aquatic race called merfolk for simplicity sake was discovered. But they weren't friendly in order to scare them off the crew that discovered them fired off a warning shot which hit and destroyed a precious religious artifact of a large tribe by mistake. Before they made there escape while humanity is surprised they don't really do anything because they have nothing to trade or offer stuck in the Stone age because of no fire to make more advanced tools so while they keep an eye on them humanity does nothing over the course of several decades the merfolk unify under said large tribe and prepare an all out war against the "above demons". They start by launching an all-out invasion of the Philippines the federated Republic of Micronesia and Samoa killing up to 300,000 men women and children in 2009 before invading the countries along the Western Pacific Ocean. Now humanity is trying to figure out how to take the fight to them and either subjugate them or wipe them out they refuse to use nuclear weapons because of the risk of poisoning the ocean.
The merfolk live 10,000 meters under water are a lot stronger and tougher than humans due to sustaining such pressure and being fine they have sonar abilities to use like ranged weapons along with being highly venomous with the population ranging around 5-10 million merfolk they have tails like eels with a humanoid top and able to move on land based on how much water they absorb. how would humanity be able to fight against such an assault from the depths.