Thunder Rift was a well-developed campaign setting for D&D of the early Nineties. This was the time of the large black box edition of Dungeons & Dragons. Thunder Rift was an isolated valley bordered on all sides by nearly-impassable mountains. It was a “points of light” setting, just a few settlements amid untamed wilderness for low-level characters to explore.
Thunder Rift deserved its own box set, but it was not to be. Dungeons & Dragons never got the bounty of fancy campaign setting boxes like its brother AD&D (Greyhawk, Forgotten Realms, Spelljammer, Planescape, and plenty more).
The Thunder Rift setting was well-supported, with a number of modules (and, yes, box sets… just not a “setting box set”).
1992 … Colin McComb … 32 pages + poster map … TSR 9350 … ISBN 1560763817
DriveThruRPG ( PDF – $5 )
Thunder Rift is a full sourcebook to the setting. Just enough info to work from, without burying the DM in minutae.
Thunder Rift with locations of the official adventures
See The Piazza’s list of official adventures for Thunder Rift. Note there is a full Thunderrift forum there, with lots of info and user-created content!