The Thunder Rift, the "other" D&D default setting

The Known World, later rebranded as Mystara, is a rather sprawling, large campaign, and choosing where to begin can be a little daunting to a DM or group just getting started, but here's an option that is ideal for those groups who just want a smaller scale map with some settlements and a wilderness in which to find adventures.

The Thunder Rift

Many Classic D&D players who got their start in the early 1990s cut their teeth on the Thunder Rift campaign, a small scale "world" made up of a large valley about 40 or 50 square miles in area. The Rift is home to a handful of settlements, as well as a fully mapped out wilderness with a few planned adventure locales and plenty of room for the DM to add his own. The trick is, Thunder Rift is a demi-plane of sorts; and though some fans have plotted a physical location in the known world for the valley, access between the realms is extremely limited to magical portals. The sheer, towering cliffs that form the valley's walls are impenetrable to mundane methods of escape.

Thunder Rift starts out with the Thunder Rift adventure and setting (TSR 9357) and continues with a short series of adventures that develop the encounter areas noted on the map in that book

Quest for the Silver Sword (TSR 9342)
Assault on Raven's Ruin (TSR 9350)
Sword and Shield (TSR 9387)
The Knight of Newts (TSR 9434)
Rage of the Rakasta (TSR 9435)
In the Phantom's Wake (TSR 9436)

Finally, DMR1 Dungeon Master's Screen (TSR 9437,black cover with green dragon art) include the mini-adventure Escape from Thunder Rift, which allows the heroes to venture out into the Grand Duchy of Karameikos (or wherever the DM wants to put them next).

The 1992 Character & Monster Assortment (TSR 9363) bears the now familiar Thunder Rift graphic presentation on its cover, but it's just a collection of fold up 3-d cardboard gaming miniatures and doesn't offer any new material specific to the Thunder Rift setting.

To those who just want to use the adventures but not the setting, the good news is that these are mostly generic adventures in the tradition of the other Basic D&D adventure modules, and adapt easily to wherever you want to place them.