This is the fanservice-in-the-truest-sense bonus you buy the book for, not a story you sit anyone down to watch. Bundled with volume nine of the novel, it exists because Takaya Kagami wrote both Itsu-Ten and The Legend of the Legendary Heroes, so Taito and Saitohimea's school-club supernatural chaos gets to bump into Ryner and Ferris's fantasy deadpan for one glorious, non-canonical episode. The comedy carries it entirely: character misadventures, the same magic-school-romance texture the source runs on, zero heavy lifting. Be honest, though, it never earns the collision the way Gintama x Sket Dance did. The two casts blur into one tone instead of striking sparks off each other. Watch it if you already love these people. Skip it if you're looking for a doorway in.</summary>}The final answer is the JSON above.</summary>{

Itsuka Tenma no Kuro Usagi gets a single OVA episode, packaged with the limited edition of the ninth novel volume and treated as an unaired thirteenth episode. Series author Kagami Takaya wrote it himself. It is made of three short stories. Two grew out of ideas fans sent in on Twitter, among them a dream of Mirai's drawn in chibi style and a body-swapping story. The third is a crossover with The Legend of the Legendary Heroes, another fantasy work by Kagami. Expect comedy, romance, ecchi moments and supernatural elements.