Kaibutsu Oujo S01E04 - Princess Negotiation

Hime, Hiro, and Flandre, with Riza in tow, travel to Hime's lake resort to relax. At night, their resort is suddenly pulled down into the lake, which has them evacuate the area. They are surrounded and then captured by a race of mermen who are after Hime's immortality powers. When a giant merman named Poseidon challenges Hime to a duel, Riza steps in and accept the challenge in her place. Although Riza struggles as she fights Poseidon, a full moon appears in the sky, giving her the strength to defeat him. The next day, Riza declares her intention to stick close to Hime in order to find an opportunity to take her revenge on one of Hime's siblings, the one responsible for making Wildman betray Hime.