The Forgotten Realms is a fantasy campaign setting for the Dungeons & Dragons role playing game originally developed by Ed Greenwood as a location for his childhood stories in the late 1960's. After publication of several short stories in gaming magazines during the mid 1980's the setting was purchased by TSR. In 1987 the first official Forgotten Realms product, Darkwalker on Moonshae a novel by Douglas Niles, was released followed by the Old Gray Box which is the original campaign setting for Advanced Dungeons & Dragons later in the same year. The rest as you say is a fantastical history of might and magic.
Widely considered the most popular Dungeons & Dragons campaign setting with many attributing this to the Late Middle Ages look and feel extensivily used in the primary materials. In terms of officially published materials there are nearly 200 novels, 80 video games, and 150 Role Playing source books.
On the fictional planet Abeir-Toril, know to the locals simply as Toril, there are eight major continents or regions; Anchorome, Faerûn, Katashaka, Kara-Tur, Maztica, Osse, Zakhara, and the Underdark. Generally speaking each of these regions has a corresponding location in Earth's history.
Anchorome is vast unexplored region home to tribal cultures similar to that of North America around the time of Viking Vinland expeditions in 11th to 13th Centuries. Beyond the nomadic tribes located throughout the continent the most populated region is the Poscadar basin with both the wild elves of the Poscadar Tribes and the human Azuposi tribes sharing the greater southwest and a Sahuagin culture called Itzcali has developed in the west coast.
Katashaka is continent located south of Maztica and as a region it is comparable to the Sub-Saharan Africa region which is poorly documented.
Kara-Tur was the original setting of the Oriental Adventures campaign setting.
In Maztica you will find a similar culture to Mesoamerica prior to the arrival of Christopher Columbus in Central America.
Osse is an island culture in the Western Sea that matches both the Australian Aborigine culture and Polonaise Traditions.
Zakhara, also know as the Land of Fate is located on a great peninsula south of Faerûn dominated by pirates and secretive cities and was used as the setting for the Al-Qadim campaign setting. Al-Qadim is a combonation of Arabian History and Hollywood Fantasy.
The continent of Faerûn with its many correlations to the Late Middle-Ages in Europe is the primary focus of the Forgotten Realms campaign setting, these subtle connections in correlation with the fact that most of the official source material are focused on this region have bred familiarity amongst the player base.
The terrain of this vast continent is as varied as its races and civilizations who reside there. Descriptions of Faerûn generally divide the continent into nine regions similar to a tic-tac-toe board; the Northwest, the North, the Northeast, the West, Interior Faerûn, the East, the Southwest, the South and Southeast.
Interior Faerûn is dominated by the Sea of Fallen Stars, a great inland sea with its many finger like bays and other irregular features. The Sea of Fallen Stars offers navigable waters allowing trade and transportation between the many states and kingdoms along its coastline. The Dragonmere [or Dragon Sea] is finger bay located at the western edge of the Sea of Fallen Stars.
The Kingdom of Cormyr is ruled by a human monarchy with its capital in the city of Suzail, a major trading port on the north coast of the Dragonmere. While still a traditional monarchy, recently the Kingdom has seen the addition of a council of advisors to assist the Monarch rule the still growing Kingdom. The Kingdom is defended by a traditional military, the Purple Dragon Knights, and the War Wizards, a faction of Wizards who use their ability to defend the Kingdom.
The Stormhorn Mountains form a natural border on the Western and Northern edges of Cormyr. Largely wild and untamed the peaks, valleys and cliffs of the Stormhorns have provided a haven for those opposed to the rule of Cormyr as well as countless beasts, dragons and demons.
The Thunder Peaks and a vast swamp form the Eastern border of Cormyr. Beneath the Thunder Peaks near the source of the Thunderflow River the abandoned ruins of the Dwarven Kingdom of Thunderholme can be found. Abandoned when the Shadow Dragon Aurgloroasa assassinated the heir to the Throne over three centuries ago rumor persists of the Dracolich roaming the empty caverns.
The city of Arabel located in north central Cormyr is the second largest city and a major trading destination in Western Edge of the Interior Faerûn and home to a large adventuring population.
Protected by the Purple Dragon Knights roads like the High Road, Starwater Road, Calantar's Road, the Way of the Dragon, and the Way of the Manticore crisscross Cormyr allowing trade and commerce to flow throughout the Kingdom.
The North and Western region of the Kingdom is dominated by the vast King’s Forest. The largely untamed forested country often ruled in name only by the Kingdom of Cormyr. Recently the activities of the Drow from Sschindylryn have made the village of Eveningstar on the North Eastern edge of the Forest an important hub of activity and adventuring.
Located where the Way of the Manticore crosses the Wyvernflow River the city of Wheloon was once a thriving city of commerce; that was before the Spellplague. After the an abnormal number of suspected Shar followers were discovered within the city the King decreed that the city be transformed into a prison sealed by brick and magic. Suspected followers of Shar, Shadovar Sympathizers, criminals and those suspected by the crown have been thrown over the walls and left to fend for themselves.
 The Underdark
Located throughout Toril the Underdark is the incredibly vast complex of subterranean caverns, tunnels, dungeons and cities that has become home to many Evil Empires and creatures drive underground during the end of the Age of Demons. It should be noted that while originally introduced by Gary Gygax as a Dungeon based campaign setting the Underdark has been adapted to be used in virtually all Campaign Settings from the Forgotten Realms, The World of Greyhawk, as well as the Eberron settings; in the Eberron setting the Underdark is called Khyber.
On Toril the Underdark is divided into three levels or regions; The Upperdark, The Middle Dark and The Lowerdark. The denizens of the Underdark often refer to the “World Above” when describing the surface. Much like continents of the world above the Underdark is divided into vast domains. The known domains under Faerûn include the Buried Domains, the Darklands, the Deepwastes, the Earthroot, the Glimmer Sea, Great Bhaerynden, the Northdark and Old Shanatar.
The Upperdark is the region located within the first three miles of the surface. It is within this region that most interactions between the surface dwellers and the denizens of the Underdark occur. The Upperdark is mostly cavernous spaces with few settlements beyond the occasional trading camp.
The Middledark is the region located three to ten miles below the surface. Within this region you will find most of the cities and other populated areas of the Underdark. The Drow inhabitants have built many city states or empires including Menzoberranzan and Sschindilryn and the long forgotten ruins of Maerimydra. The Duergar, Kuo-toa, Svirfneblin (Deep Gnomes) are a few of the other races that have built empires within this region.
Located more than ten miles below the surface the Lowerdark is the region that even the denizens of the Underdark fear to go. The Lowerdark believed to the home of powerful demons and other abominations driven underground at the conclusion of the “Age of Demons”.
 The Drow City of Sschindylryn
Located miles below the surface of the King's Forest in Cormyr on the shores of Lake Daerbraun the Drow city of Sschindylryn is built upon the ancient ruins of Kuo-Toan city. Sschindylryn is dominated by the central ziggurat rising nearly 1/2 a mile above the city with the Temple of Lolth at its apex.
Lake Daerbraun, sometimes refered to as Lake Thalmiir, is massive underground lake or small sea fed by a massive waterfall draining from the Plunge Pool on the surface located deep within the King's Forest.
Sschindylryn was once the center of a large war-like Empire. Using the magic portals and gateways for which the city is famous, vast raids and war were fought throughout numerous domains of the Underdark. Sschindylryn came into conflict with the Empire of Menzoberranzan, another powerful Drow city state and was throughly beaten in battle reducing Sschindylryn to a minor military power in the Underdark.
 Integration into Dungeons & Dragons Online
 Menace of the Underdark
Menace of the Underdark was announced on January 19, 2012 by Fernado Paiz, Executive Producer of Dungeons & Dragons Online: Eberron Unlimited as the first premium expansion to Dungeons & Dragons Online: Eberron Unlimited. The Menace of the Underdark is a part of Update 14
With the inclusion of Forgotten Realms setting the official name of the game was changed to Dungeons & Dragons Online. Senior Producer Eric Boyer in an interview with GameBanshee commented "Yeah, starting early this year we’re going to be Dungeons & Dragons Online. That’s also something that we had taken some time and had a lot of discussions with WotC about how we represented that. So we’re officially going with Dungeons & Dragons Online as our primary title and then we’ll add the subtitles when it’s appropriate, for example, "Menace of the Underdark" for the expansion."
In an interview with GameBanshee Fernando Paiz states "So we probably started talking about this with them about six months [March 2010] after we went free-to-play where we really started seeing the success as something that was provable, that was going to keep growing the franchise, and then frankly had legs." Development on the new expansion began in late 2011 with approval of both Warner Brothers and Wizards of the Coast to "invest millions of dollars into the game".
 The Story Line
From the character's perspective the story line for the Menace of the Underdark actually begins with the Update 13 "Web of Chaos," a three quest story arc in which the characters investigate the activities of the Lords of Dust underneath Stormreach Harbor. The “Menace of the Underdark” expansion introduces players to another campaign setting, the Forgotten Realms. The expansion takes place in the Kingdom of Cormyr, located on the continent of Faerûn, the Underdark, the Drow city of Sschindylryn and in the Demon Web.
 Shadowfell Conspiracy
The Shadowfell Conspiracy is the second premium expansion to Dungeons & Dragons Online, announced February 26, 2013 by Erik Boyer (AKA Senior Producer Glin) in his | Happy Birthday Letter on the DDO Forums. The Shadowfell Conspiracy will be a part of Update 19.
The Shadowfell Conspiracy brings two new regions to DDO; the prison city of Wheloon and the Stormhorn Mountains.
 The Story Line
Update 18 Disciples of Shadow begins the storyline for the characters. A three quest story arc in which characters investigate the activities of the Shar in the Stormhorn Mountains and in the Wheloon Docks. The first two quests in the story arc are free to play; however the third quest requires the purchase of the Shadowfell Conspiracy expansion.