The Lost Media Wiki’s article on the subject!
The only known screenshot in existence
Fire Emblem: Maiden of Darkness was a game in the Fire Emblem Series that was supposed to be released for the N64DD.
Information
In a 1998 issue of 64 Dream, a page advertising a new Fire Emblem was supposed to be released. The game was supposed to be for the Nintendo 64 and its variant N64DD. It was first revealed in 1997 by Shigeru Miyamoto. But due to failing sales, structural changes at Intelligent Systems, and the creator Shouzou Kaga left the company to form Tirnanog Co. after Thracia 776 (Which caused to lose the rights of the series due to legal issues with Nintendo about Tear Ring Saga infringing their IP), the game was cancelled in 2000. In December of the same year, 64 Dream confirmed the cancellation. Many elements of Fire Emblem 64 were later incorporated into Fire Emblem: Binding Blade for the Gameboy Advance. Similary, a third Fire Emblem game for the Wii was planned but cancelled, due to struggles with the developers, in addition to an unfocused development schedule and the poor sales of the console. The game, now subtitled Ankoku No Miko, remained lost and obscure to this day.
Videos
Gaming Mysteries- Fire Emblem 64 - Pocket Monsters RPG (N64) Cancelled
Yuriofwind's video on Fire Emblem: Maiden of Darkness (64) and Pocket Monsters 64.
The N64's Lost Fire Emblem Game - Lost & Obscure Nintendo -3
Sakura Stardust's video on the subject.