Full Moon

The Full Moon (満月, Mangetsu) has a powerful effect on liminals; making their carnal desires more pronounced and reducing their self-control.

During the chapter, this combined with the possibility of Kimihito being able to marry one of them caused the girls to lose their control and use their strength in full, to the point where he feared for his life.

Amusingly enough, the way to apparently reduce the effects is to literally cool their heads. The morning after, it was shown that the girls have little to no recollection of their behaviour while under the effects of the moon.

After learning of this firsthand, Kimihito determined that his home stays must go to bed early on nights with a full moon, and has apparently not had another incident since. However, in reality the girls (Miia, Centorea and Rachnera) were constantly affected by the moon and driven to sleep with him, but had worked out a system to keep each other in check and if that didn't work out, they arranged to have Suu stop them before they entered Kimihito's room.

Trivia

 * This is an obvious allusion as to how animals and people are supposedly affected by the full moon, bringing out their more bestial and primitive nature.
 * The words "lunacy", "lunatic", and "loony" are derived from "Luna" because of the folk belief in the moon as a cause of periodic insanity.