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.
Marin and Kanon saving Urin from Sedna together, even after their powers have been taken away.
Instead of defeating Sedna with a spear of light and killing Urin in the process, Marin and Kanon reach out to her. They hug her, and the power of Marin's love and Kanon's acceptance of darkness moves Sedna's heart and causes her to disappear peacefully.
The aesop of the series. Everyone has darkness in their hearts. That's okay, and we must learn to accept it, not cast it out.