The Spell Plague

The False Goddess Mystra's death sent ripples of magic through the weave (the source of arcane magic) and caused devastation throughout the region. To the south, the land became scarred, warping creatures hideously, and creating a blistering magical heat that has lingered for centuries. To the north, the land was coated in a frigid landscape of unnatural cold and roaming storms of psychic energy.

Towards the end of the Age of Conflict, the Goddess of Magic, Mystra (of the previous pantheon of gods), was killed. The sheer wave of magical power collapsed the Weave and sent echoes into the material plane. Magic users all over the world were effected. Some lost their ability to cast spells, others went mad, and some still were killed outright.

In the end, waves of destruction traveled across the landscape of Halruaal tearing the northern and southern reaches up:

The north became a tumultuous region of stinging cold. To the northeast and endless winter fell and a frosty mist shrouded the majestic floating islands. To the Northwest, severe storms of ice and lightning have sprung up and continuous to rage to this day.

The south became a blistering region of near unparalleled heat. The Sekhemti was not always a desert, but within a second, a once thriving ecosystem became a sand filled wasteland. Settlements once founded by House Kamose were destroyed and buried. Many of them now lost to the ages.

The midlands, while not completely unaffected, were left relatively unharmed. An endless whirlpool, some 50 miles in diameter formed off the northwestern coast of the Heartlands and disrupted any air travel over it with battering winds. "The Maw" has effectively cut off all travel in that 50 mile block. In addition, a metaphysical mutation sprang up in many of creatures in the inner sea, adding to their already dangerous abilities.