From the Forgotten Realms Campaign Setting book:

Toril is steeped in magic. It permeates the entire world. Fallen empires thousands of years old left portals and wrecked towers scattered across the landscape that are still filled with potent enchantments. Haughty wizards whose spells can lay low entire armies plot against each other as they pursue their studies into ever more powerful – and more dangerous – fields of arcane lore. Deities channel divine energy through their mortal agents to advance the causes that interest them. Adventurers of all types, evil and good, wield mighty spells seemingly at will.

Most Faerunians never learn to speak a spell, but magic touches their lives in ways they do not always see. Skilled wizards and sorcerers serve the monarchs of the land, plying their spells to defend their realms against attack and to watch their enemies’ movements. Clerics intercede with the deities to invoke their blessings as real and tangible benefits to the endeavors of the community. Monstrous aberrations of twisted magic and warped energy are often the deadliest creatures to prey on Faerun’s common folk, and adventurers armed with enchanted steel are the lands first line of defense against such perils.

The history of Faerun is dominated by the cyclic rise and cataclysmic destruction of empires founded on knowledge of the intricacies of magic. The Imaskari wrought magical portals to bridge the gap between worlds, only to be destroyed by the god-kings of the slave races they imported to Faerun. Their lost realm now lies beneath the dust desert of Raurin. The mighty Empire of Nethril dominated the center of the continent, its skies graced by floating cities and its wizards commanding unimaginable might. They reached too far and were destroyed in a magical catastrophe of world shaking proportions, forever changing the workings of magic itself. Realms such as Narfell and Raumathar, Athalantar and Cormanthyr, Illefarn and Hlondath have left their ruins throughout the world.

Magic both old and strong still slumbers in the wreckage of these ancient realms. Every year some new marvel is rediscovered in an old ruin: a spell never before seen or a wondrous item of great power and high purpose. More often, though, blights and perils long forgotten or magical abominations that should never see the light of day emerge to trouble the world anew, unearthed by those ignorant or unscrupulous enough to seek them out.