- Some of the songs can create Soundtrack Dissonance to creepy effect, such as a calm tune playing over shots of a dying, empty ocean.
- The eclipse heralds eerie blue butterflies and turns the moon red, ending with Urin's corruption,. It's signified by her eyes turning red and eventually growing butterfly wings along with gaining destructive powers, and she tries several times to kill Kanon out of jealousy.
- Two monsters of the week have Prehensile Hair for weapons and use then to strangle Kojima and nearly crush his neck. The corrupted Urin wants to make Kanon feel despair by killing the person she likes, and she nearly succeeds.
- Sedna corrupts the islanders by giving them Volcanic Veins, glowing red eyes, and making them act horribly to one another, with Sam, Ichikawa, and Warin being brainwashed to kill Marin, the Elder Turtle, and Kanon.
- Sedna's brainwashing of Urin involves convincing her that no one will come for her, no one will love her, and that it's better to shut down rather than feel anything—by allowing Sedna to inhabit her, she would be released from all sorrow. Not only is it an awful thing to do to someone, it's also what many may feel when in a state of severe depression—feeling unloved and wanting to shut down rather than feel anything. Sedna then turns the situation against Marin and Kanon as well, as killing her would kill Urin, her host.
- In episode 11, Marin and Kanon try to combine their powers to save Urin from Sedna. She just absorbs it, draining them completely and shattering their transformation stones, completely eradicating any chance of stopping her by force. It's set to a dark reprise of their transformation music.
Nightmare Fuel / Umi Monogatari