Isle of Dread
The Isle of Dread is a primeval, monster-infested jungle island in the Dungeons & Dragons multiverse, loosely based on a combination of Skull Island from King Kong (though not to the same extent as the Isle of the Ape) and Jurassic Park. A prime "pulp fantasy" location, it was the centerpiece of its own BECMI module, "X1: The Isle of Dread", and introduced many unique monsters and races, such as the aranea, phanaton and rakasta. It returned in subsequent editions and has become an integral part of the D&D lore, aided by the fact it was packaged as a free inclusion in the D&D Expert Set, so anybody who bought that rules update in the early 1980s was exposed to the Isle of Dread. It was ranked the 16th greatest D&D adventure module of all time.
Written by David "Zeb" Cook and Tom Moldvay, the module X1: Isle of Dread first debuted in 1981, and was available both on its own and packaged into the Expert Set as an example of an outdoors-based adventure/setting. This original version is laid out in the style characteristic of early D&D adventures: it had no Dungeons & Dragons logo, a diagonal strip in the top left corner indicated which edition of the game it was for, and the back cover featured an illustration and a list of other D&D products of the time. A reprint was made in 1983; this second version was also packaged with the revised version of the Expert Set, and can be distinguished by its Timothy Truman cover art, the red-orange border on its cover, and its use of typical mid-1980s D&D layout elements: the game's current logo was prominently featured on the cover, the diagonal strip was replaced with a horizontal one across the top, and the back cover featured no illustration but did have a text description of the adventure. There are a few minor differences besides appearance between this and the earlier version, including the replacement of a few monsters, and a mapping error that makes part of the final temple appear to be completely inaccessible.
X1 came with a fairly detailed (for its time) map of a setting then called the Known World, showing at least fifteen distinct nations on the mainland to the north, as well as much of the Sea of Dread in which the Isle of Dread could be found. These nations each received a paragraph or so of description near the beginning of the module, making this the debut of several prominent Mystaran nations - Darokin, Karameikos, Ylaruam and Thyatis.
Described as a medium to high level scenario, X1's plot was simple, even for the standards of the time. The characters somehow find a fragment from a ship's log, describing a mysterious island on which many treasures can be found, and set out to explore it. Typically, the characters will first make landfall near the more or less friendly village of Tanaroa and after possibly dealing with some troublesome factions in the village, set out to explore the interior of the island. In the course of their explorations, they may find a number of other villages of unfamiliar intelligent creatures, numerous hostile monsters and the treasures they guard, and a band of pirates. Many prehistoric creatures, including dinosaurs, are prominently featured, especially in the original printing of the adventure. Near the center of the island is a hidden temple inhabited by monstrous, mind-bending creatures known as kopru; the characters may stumble across it or learn that it is a source of problems for the other inhabitants of the isle, and the climax of the adventure typically consists of the characters exploring this temple, battling its inhabitants, and uncovering its secrets.
Returning to the Isle of Dread
The Isle is also a minor encounter area in the later adventure Lathan's Gold, and receives some further mention in several later D&D products such as the Poor Wizard's Almanac series.
Issue No. 114 of Dungeon Magazine features an update on the Isle of Dread as a Greyhawk setting, a remake-sequel to The Isle of Dread entitled Torrents of Dread, and a poster-style map of the island, as well some smaller surrounding islands. In this update, the island was located in the Densac Gulf, a region bordered by the Azure Sea to the north, the Pearl Sea to the south, the Amedio Jungle to the west, and Hepmonaland to the east. This large expanse of ocean contains several island chains, one of which is the Isle of Dread itself. The update details a kopru plot that destroyed the city of the original Olman settlers through the power of a giant black pearl imbued with the influence of Demogorgon, the demonic god of the kopru. The isle has become a mad collection of kopru, other aquatic races, demonic beings, dinosaurs, and savage Olman natives.
The Savage Tide
The Isle of Dread made a prominent return towards the end of 3rd edition, as it was the foundation for the first half or so of the Savage Tide adventure path. This version of the Isle of Dread is, like the earlier Torrents of Dread adventure, set in Greyhawk rather than the original's Mystara, but still retains some Mystaran roots, like the presence of the aranea and phanaton races.
Across the Planes
In 4th edition, the Isle of Dread is stated to be a planeshifting island, moving freely between the material plane and the Feywild. 5th edition retained this idea but moved it to the Elemental Plane of Water instead... which is a plane that consists of an infinite underwater expanse. Except this actually makes sense in 5e, because now the inner bits of the Plane of Water have a surface with islands. What happens as you go further out is anybody's guess.