As a boy, the legend goes, Nerull was alone in a wood when he saw the image of Beory herself in a small clearing. Around her attended a flock of animals. Nerull stayed hidden and watched from afar. The Nerull's father stumbled into the clearing calling for his son. He was completely startled to come upon the great goddess and stared in awe. All the animals scattered, causing Beory great sadness. She walked up to Nerull's father and touched him with her finger, then disappeared. Nerull ran to his father's crumpled side, to find him dead. Raw necromantic energies remained, and these surged into Nerull. He buried his father at this site.
From that point on, Nerull could know the time, place, and manner of any creature's death by looking upon it. He fled Flan society and lived in the wild, courting his necromantic gift. He learned the raw power that could be gained from a dieing creatures life essence. Nerull skirted the edges of society, sneaking in while someone was dieing. He developed an evermore ghastly appearance, and those who did see him were frightened and fled. Nerull created his dread scythe during this time.
By -3175, word of Nerull had spread across practically all Flan lands. He strove to be at all Flan deaths, growing greatly in power. Flan came to believe that he was the cause of all death and started leaving offering to stave him off. If he accepted the offering, Nerull would use his powers to heal the person's ailment.
In -3150, it came that the two largest Flannish families went to war. Their armies battled in a field not far from where Nerull's father was buried. The Reaper appeared and slaughtered all the outlying soldiers of both armies. He ordered those in battle to continue until one army was victorious. Desperate men fought each other as Nerull danced among them, slashing the cowardly and dieing with his scythe. Amid the blood and gore emerged 30 victorious soldiers. Nerull demanded they follow him or be obliterated forever. Vast necromantic powers surged through the survivors. The became the Masters of the Shroud, completely dedicated to Nerull.
The Masters spread across Flan lands, ordering them to placate and worship or die. Nerull became a demi-god on Oerth. Within 50 years, practically all Flan followed him. He crowded them together, looking to eliminate the nomadic nature. A sedentary people are easier to control. Agriculture flourished as the means of survival, and the population ironically boomed under command of a demi-god of death. By -3075, his societal reengineering had been completed, nearly all living within 50 miles of the Sheldomar. His Masters of Shrouds ruled over armies of Flan, raising undead legions to compliment the living.
Nerull envisioned himself ruling over the elves. Lands eash of the Sheldomar looked empty and undefended. Flan and undead armies crossed the river some 50 miles north of present day Niole Dra. The scarttered elves fled from this unholy host. Celene arranged a guerilla war but would not meet the host in mass battle. Then Nerull plunged his army into the Lortmils to destroy the great elvin cities. Tirillifair was the first. This grey elf city covered an entire mountain and was well defended. Several waves of the horde were repulsed before the dark lord settled on a seige. Celene finally poured all its energies into an army that assaulted the beseiging horde. The massive battle lasted days, and included Nerull's forces assaulting and capturing Tirillifair in the confusion. Nerull's unparalelled ability to raise the dead is what kept the elves great magical abilities from eliminating his host. Their magics were winning out, however, and Nerull eventually found himself trapped within the city he captured, beseiged by the elves. In madness, he ordered the city completely destroyed, with the remaining army to charge the elvin forces. The resulting battle was fierce beyond any that had occured before or since in the Lortmils. The Masters of Shrouds were all destroyed, and Nerull material body finally obliterated. This final act caused necromantic energies to enswirl the entire mountainside, haunting the land still today. Nerull rose to become a true god of the outer planes, forever to torment the Flan people.
According to this the Flan should have been wiped out. IF Nerull collected them all and then failed in his war of Conquest, whose left to repopulate the race?
You have a haunted Mountain in the Lortmills, I've never even heard a rumor of such a thing existing. Maybe the Drachensgrabs, Sure, but the Lortmills are as safe as range as the Flanaess have.