Deimos no Hanayome: Ran no Kumikyoku revolves around high school girl, Minako Ifu, who has become the focus of the attentions of Deimos, former god of terror. For the forbidden love between him and his younger twin sister, Venus, the former goddess of love, he was turned into a demon by Zeus, and she was imprisoned in the Swamp of Death. In order to rescue Venus from her fatal punishment, he has to bring Minako, who appears to be her physical reincarnation, to the underworld so she can possess her body. 
 
Adapting a chapter of the manga, this OVA is centered on Minako visiting the mansion of an infamous botanist in search and investigation of her missing friend.

Deimos was once a handsome god. He loved a beautiful goddess who returns his sentiments. The problem, well, she is his sister. For their crime against nature they were struck down out of Olympus. The brother is now a demon and the sister a rotting corpse at the bottom of the ocean. Deimos must choose between his sister and her living human incarnation. His sister is jealous. The girl is horrified and unsure of just what to make of her situation.
 
(Source: AnimeNewsNetwork)

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