The Isle of Dread is the first X-series (eXpert level) adventure and also the first D&D wilderness adventure. The party finds a map to a fabled island, and must journey from Specularum in the Grand Duchy of Karameikos (not detailed in the module) across a hazardous sea to an remote archipelago. The isle itself has a King Kong-style native village separated from the bulk of the land mass by a narrow isthmus and a wall; and a Lost World feel with dinosaurs and lost civilizations.

This is the first product from TSR to describe the Known World (later, Mystara), providing a map and brief summaries of the major nations. Many new monsters were introduced, including a wide variety of prehistorica fauna, the phanaton (flying squirrel-like humanoids), rakasta (cat people), and kopru.