It was called Nehrim, and the guys who made it are about to release a similar mod for Skyrim called Enderal. I wish more people would make mods like this, kind of like how Neverwinter Nights had all those self-contained modules made by the community, many of which were set in their own universes separate from the D&D setting. I know this doesn't happen because Bethesda games require more effort to mod, but it's fun to imagine the possibilities.
BTW, the technical term for this kind of mod is "total conversion".