It was to be a 2D Castlevania title for 3DS and was in development for about 18 months, with 2 player co-op a feature. It would have worked with a Wii U version but that was ditched.
It could still be revived though, reports the, the guy who brought us news of a PlayStation All-Stars Battle Royale in the works. Mirror of Fate uses the 3DS cameras.
The forward-facing camera acts as the mirror while the outward facing one is for puzzles. A similar setup would have been used with the Nintendo Wii U version. However Konami put those plans on hold and decided to give their full attention to porting Castlevania: Lords of Shadow 2 to Wii U instead.
Spanish dev Mercury Steam are creating the two titles and they'll be officially unveiling Castlevania: Lords of Shadow 2 and Castlevania: Mirror of Fate next month at E3 in California. The Wii U port will retain some of the 3DS connectivity.