Cult Classic: Although a hit when it was first released, Morrowind was eventually eclipsed by its sequels in popular eyes (arriving at Stage 6a of the Fandom Life Cycle). Nonetheless, it is still actively played and modded by a fiercely dedicated and very militant fandom core to this day.
Killer App: This game was one for the Xbox for those fans who didn't buy one for Halo, being one of the first then-modern Western RPGs available on a console.
Multi-Platform: For both Xbox and PC, making it the first game in the series to be multi-platform. This allowed the game to get into the hands of a wider audience, helping make it a Breakthrough Hit for Bethesda and introduce a massive Newbie Boom.
Sequel Gap: The six-year gap between Daggerfall and Morrowind has been the longest between games in the main series to date. (Though by now the gap between Skyrim and The Elder Scrolls VI is going to be even wider.)
Urban Legend of Zelda: There were supposedly a number of ways to join Dagoth Ur and/or the Sixth House, most of which were debunked rather quickly. As you can see in the below entry, this one was almost a real part of the game...
In very early planning, this game was almost called Tribunal and would have taken place in the Summerset Isles, homeland of the Altmer. Some preliminary artwork including cover art were produced in 1996 after Daggerfall's completion.
Original plans for the game had it taking place within the entire province of Morrowind, rather than just the island of Vvardenfell. This would idea would be scrapped in the early development phase.
Word of God: Or Word of Dante/Word of Saint Paul, depending on how "canon" you consider the works to be. Some of the game developers have posted "obscure texts" on the official forums regarding a number of in-game topics and characters. They are generally regarded by the fandom as canon. Michael Kirkbride, in particular, wrote a number of them which fill in gaps around The Tribunal, Vivec, and the Dwemer. (Kirkbride is also the author of many of the in-game books, including most-famously the 36 Lessons of Vivec series.)