Longest crossover I've ever made, so if I was lazy sometimes, get over it.

Plot: A flashback of Hades and Ursula trying to destroy Atlantis, just because they can. It fails however when a mysterious light creates a forcefield - and Ursula is killed in the process. Cut to the present day when Hades is still upset over this and can't seem to understand what the light and Kida have to do with each other. The fates show him a random mermaid passing by and Hades decides to use her as a spy to get into Atlantis. Ariel, of course, isn't happy about this but when Hades shows her dead girlfriend and says he can bring Meg back, she agrees. Things don't go according to plan however, as Kida and Ariel form a strong bond. Hades sees this and to speed things up, brings Meg back and pretty much tells her to get Ariel moving along or he'll turn her into a polyp.
Hades summons Ariel back down, her and Meg are reunited, he gets impatient and when Ariel hesitates to sell out Kida, Meg seduces her into talking. Ariel tells Hades all about what she saw in the city and how the crystal works. While she is feeling guilty, he tells her that Meg was working with him and Ariel feels betrayed. He goes to the city but the girls beat him to it and try to warn Kida - a little too late. Ariel tries to apologize but Kida will have none of it. He tries to destroy the city, Kida transforms, Hades flees and in the end, they all go their separate ways.