Moonshae Isles
The Moonshae Isles are a region of the Forgotten Realms that can be best summed up as "the British region". Like, Faerun as a whole is vaguely European in nature, but the Moonshae Isles crank the British mythology meter to the red and don't look back. Legend has it that they started out as an entirely separate setting, but were borged into the Realms, similarly to Kara-Tur.
The Moonshaes are a roughly circular archipelago of subarctic islands that lie upon the Sea of Swords, west of Amn and southwest of the Sword Coast. Dominated by a mixture of moorland, mountains, and thick forests of both deciduous and coniferous trees, the weather is characterized by general low temperatures, harsh winters, and abundant (often severe) storms. The abundance of moisture leads to plentiful rivers, swamps, lakes, marshes and streams.
The dominant race of the Moonshaes are, of course, humans, which are divided into the more civilized "Pagan British" themed Ffolk and the Northlanders, who inhabit the northern parts of the archipelago (hence their name), may be distantly connected to the Uthgardt barbarian tribes, and are one of the Realms' many not!vikings civilizations. The Ffolk are adherents to a druidic religion that worships a deity called "The Earthmother" - said to be an older, more fey aspect of Chauntea in most editions, briefly retconned into one of Toril's Primal Spirits in 4th edition. The Northlanders worship a pseudo-Nordic pantheon made up of Auril, Tempus, Umberlee, Valkur and Talos. Neither groups is particularly welcoming to missionaries from other lands.
Demihumans of the Moonshaes consist of dwarf and halfling clans, and the Llewyrr; a local elf descended from high elves (or eladrin, in 4e) that fled the Llewyrrwood (what is now called Neverwinter Wood) thousands of years ago, only to be stranded on the Moonshaes. They are a dwindling race, faced with the simultaneous dillemmas of infrequent births (even by elf standards!) and the fact they produce far more daughters than sons, which has resulted in the race becoming almost elven amazons, as the women have had to take over the martial protection of their lands. They are strict vegetarians, and particularly revere trout as vessels for the spirits of their ancestors. The Llewyrr's fortunes received an upturn after the Spellplague, when the Isles became temporarily linked to the Feywild and this led to the materialization of Eladrin feyspires, which led to both magical and mundane infusions of life into the formerly dying elves... what happened after the Sundering is currently unknown.
Firbolgs are present on the island and, despite normally being a benevolent race in the restof the Realms, have largely been considered an evil, warlike race on the Isles... which was mostly to do with their being conquered by Kazgoroth, a demonic beast-god said to be an aspect of Malar. During the unnamed(?) age after the Spellplague, fomorians established a foothold in the Isles, and frankly made a superior replacement for firbolgs in the role of "evil giant-kin".
Song Dragons are the only known native species of dragon, and some believe that this is actually their birthplace.
The Moonshaes had their own dedicated splatbook in Advanced DUngeons & Dragons 1st Edition, called, simply, Moonshae. They were also featured in the Moonshae Trilogy, and its sequel the Druidhome Trilogy. After that, they largely were mentioned in passing in Forgotten Realms campaign books for future editions. Dragon Magazine #362, one of the last "pre-4e" Dragons, contains an article providing a history of the Moonshaes as they stood in 3rd edition. Dungeon Magazine #196 provides an in-depth outline of the Moonshaes as they stood in the post-Spellplague Realms, and Dragon #405 offers Moonshae-specific character themes and the Llewyrr Elf as a playable race.