Plaguelands

The Plaguelands is the area that was once comprised of the kingdoms of Unther and Chessenta.
Unther was an Old Empire formed by slaves who escaped the control of the Imaskar Empire. It was a primarily human nation and was originally benevolent. The land was ruled for centuries by a single leader, the God-king Gilgeam. It is said he claimed he was a demigod. Under the leadership of the God-king Gilgeam the land grew into one of the worst tyrannies on Faerun, and despite having a large population of former slaves, they turned into the very thing they had been trying to escape. Slaves in Unther were either members of those families who fell from Gilgeam's favor, or members of non-human races. Their lives were worthless in the eyes of those in power, and they were typically bought from other nations, or given slavery as a sentence for crimes, imagined or real. The country had began to change for the better following the death of Gilgeam in 1280 DR, after which the slaves and the middle class overthrew the nobility. The nobility that survived their overthrow became fugitives in their own land. Slowly the land began to change for the better, slaves were outlawed and the citizens began to prosper. This was cut short with the spellplague. Out of their population of around 4,000,000 citizens, only a few hundred survived the devastation of the spellplague.
Chessenta was a confederation of city-states bound by common culture and mutual defense, but for most of its history wasn’t truly a unified nation, and the Chessentan cities were often at war with one another. Chessenta was one of the so-called Old Empires of Faerun. The Chessentans were a passionate people, always living to the fullest and never doing things half-way. Seen by outsiders as a drunken and riotous people, Chessentans had a well-earned reputation as fierce warriors and cunning tacticians. While they appreciated the fine arts and theatre, the culture of the Chessentans revolved around war and its heroes. This dedication to war suits the Chessentans well, for their nation has prospered through the conflicts between their rival cities. The title of War Hero was one of the greatest honors a person could earn, and usually any particular battle only had one person worthy of such a title. Few nations in all Faerun were as erudite in philosophy, astronomy, poetry, and history as Chessenta. Theatres and libraries were commonplace, and even in smaller cities such poetry and philosophy were practiced in public squares. Chessentans also loved sports and their athletic competitions were popular events, particularly wrestling, and almost every citizen had some skill at fighting. Chessenta’s sizable middle class controlled the country’s economy, with a government that encouraged prosperity for all, so tax revenues were high. Despite such progressiveness, Chessentans practiced slavery, and most of their slaves were elves, although this practice was less widespread than in the nations of the Old Empires, and a slave-owner could grant a slave freedom at any time. In the 15th century DR, Chessentans also developed a deep-seated mistrust of all things arcane. Chessenta fared better than Unther in the devastation of the Spellplague, out of their population of almost 3,300,000 a few thousand survived, most of these survivors fled west and became some of the first citizens of Vilhon Reach.
The Spellplague distorted everything—stone, flesh, and even the spirit. Creatures affected by the Spellplague were warped and transformed into aberrations, although every creature manifested those changes differently, and thus plaguechanged creatures differed greatly in form and abilities. Because of their violent transformation, all plaguechanged creatures lost their sanity during the process. Along with the plaguechanged the residents who died ad a result of the Spellplague were bound to the land, forced to wander as ghosts and undead. During the first few years after the Spellplague, any creature who entered the Plaguelands would be afflicted by the effects of the Spellplague, but as time passed those effects lessened. Now as long as one doesn't spend extended periods of time within the lands they won't be affected.
During the Spellplague, the Abeiran nation of Tymanchebar was dropped onto the region of Unther. The surviving Dragonborn founded the kingdom of Tymanther in tribute to their lost brethren, and to the kingdom they believed they had accidentally destroyed. Due to the hardy nature of Dragonborn, they were unaffected by the Spellplague and were able to protect their stronghold within the cursed land.
Not many dare enter these haunted lands, and Vilhon Reach has forbidden any passage along their eastern border. They have set up a walled defense to protect their nation from any wandering abominations that come from the Plaguelands.

The plaguechanged took on many different forms. Souls were twisted, torn apart, and joined together to create abominations. Some were harmless and merely wandered the land, while others would have violent tendencies and were known to attack anything on sight. These poor souls were once the inhabitants of the lands of Unther and Chessenta that were twisted by the Spellplague. Some even slightly resembled their original form, but most were mutated beyond recognition.

Plaguechanged creatures gained magical abilities, some of those were blue, fire based abilities, and others were abilities that warped the reality around them. No plaguechanged creature fully understand their new power though, and it tends to be unpredictable, fluctuating with the raw emotions of the creature.