いつか天魔の黒ウサギ OVA

A Dark Rabbit has Seven Lives OVA

According to the official website, an OVA of Itsuka Tenma no Kuro Usagi was announced to be bundled with the ninth volume of the novel. An animated crossover between Itsu-Ten and The Legend of the Legendary Heroes (Densetsu no Yuusha no Densetsu or DenYuuDen) fantasy story.