Deimos no Hanayome: Ran no Kumikyoku

A high school girl named Minako Ifu becomes the target of Deimos, the former god of terror, who was transformed into a demon by Zeus for an illicit love with his twin sister Venus, the former goddess of love. Venus was imprisoned in the Swamp of Death as punishment. To save her, Deimos needs Minako, who seems to be Venus's reincarnation, to accompany him to the underworld so she can take over Venus's body. The OVA follows Minako as she goes to the mansion of a well-known botanist to look for a missing friend.