What was Ura Zelda?
Ura Zelda was planned as an expansion for Ocarina of Time on the 64DD, Nintendo's disk drive add on for the Nintendo 64. The design was a patch in the modern sense: the cartridge stayed in the console, the disk added a second run through the game with rearranged, harder dungeons.
The project carries a famous side effect. Eiji Aonuma, assigned to Ura Zelda, disliked the work, and the bargain struck with Shigeru Miyamoto, one year to make a new game instead, produced Majora's Mask. The expansion's loss is woven into the series' strangest game's existence.
The 64DD failed commercially in Japan, and Ura Zelda's release died with it, even though Miyamoto stated in August 2000 that the game was finished. It finally shipped on GameCube as Master Quest: in Japan on November 28, 2002, under the GC Ura name, and in North America in February 2003, as the pre order bonus for The Wind Waker there and bundled with it in Europe.
Whether Master Quest is all of Ura Zelda remains an open debate among fans, who note the shipped game's size and the absence of once promised additions. This page records the debate as a debate: what is certain is the disc in hand, a finished expansion that outlived its own hardware.