Nedra Gorge lies in the northern part of the Karen Valley, across the Clearwater Lake. A small fishing villiage used to lie at its mouth, but this was destroyed some time before the Battle of the Blizzard--supposedly by a black dragon, though reports differ. No one has attempted to rebuild the town, so it has been taken over by the nastier creatures of the gorge.
In the depths of the gorge lies the ruined city of Nedra. Undead of all sorts roam both the city and the gorge. Beneath the city there are said to be many tunnels and passageways, collectively called the Lichdoom, since they are said to be the home of a Demilich.
Nedra was founded by survivors of Netheril, led by a mage of great power (his name is not known). They settled in the Karen Valley not long after the glacier had receded. Evidence of their civilization can be found in places throughout the valley, but the only substantial remains are in the Nedra Gorge--the city of Nedra itself.
The society they built up only flourished for a few hundred years, before falling prey to a strange curse. It is said that when the mage who had founded the city was nearing the end of his magically prolonged life, he consulted the priests of the god Kozah, and asked them how he might prolong it yet more. They took council among themselves, and suggested to him a ritual, which would give both him and themselves everlasting life. An oracle was consulted about the wisdom of this ritual (which was marketed as mutually beneficial to all), but though the answer was unfavorable, the ritual proceeded.
The results can be seen today: the gorge is filled with perpetual gloom, and the remains of the guards and civilians of Nedra wander it as undead. The mage remains as a demilich far down in the caverns beneath the city--known as the Lichdoom--and a few ghosts haunt the city itself.