The Undine King has been horribly mutated by the toxin in the lagoon. With his mind gone, the king attacks Jen and Scree. Having been locked in the main generator room with him, they have no choice but to fight him!
This is a strange battle. You are in a cylindrical room with the boss in the middle. You are free to swim anywhere within the confined 3-dimentional environment. Scree, your AI controlled, and playable partner, is running around a platform that runs along the walls of the room. Along the walls are valves that will open the hatch in the top of the room, allowing you access to the controls that will turn the generator on. In order to turn the valves, Scree needs Jen’s help.
Between turning the valves, you as Jen have to fight the Undine King to keep him from attacking you, otherwise his attacks disrupt you as you and Scree try to turn the valves. Once you’ve attacked him to the point of unconsciousness, he’ll spend a few seconds recovering his energy, giving you time to open a few valves. Once all the valves have been opened, the toxin in the room is removed.
It seems that the toxin was regenerating the King in the recovery periods, and now that the toxin is gone, you can kill him now.
After beating the Undine King, Jen has remorse for killing Aino’s husband, as it was him Jen and Scree were to help. Aino, via psychic telepathy, assures Jen that what she did was the right thing, and that she holds no grudge against her for doing so. She tells Jen that what she killed was not her husband, Adaro, but a horrible monster that took over him. Her husband was already lost.