Forgotten Realms

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 the Moonsea 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.

Geography
On the fictional planet Abeir-Toril there are eight major continents or regions; Anchorome, Faerûn, Katashaka, Kara-Tur, Maztica, Osse, Zakhara, and the Underdark. Generally 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.

The continent of Faerûn with its many correlations to Europe in the 14th and 15th Centuries is the primary focus of the campaign setting, these connections have breed familiarity amongst the player base.

Katashaka is a sub-Saharan region located south of Maztica 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 is located on a great peninsula south of Faerûn dominated by pirates and secretive cities.

The Underdark is a complex underground cavern system located throughout Abeir-Toril home to many Evil Empires and creatures drive underground during the end of the Age of Demons.

Integration into Dungeons & Dragons Online
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.

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."

Development
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 The 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.