The Deadwastes

Before around 350 years ago, the Barrens also encompassed the area that is today known as the Deadwastes. This was a grim, ashen, embattled land in which great human armies waged and won wars against dragons, elves, and many other forces for evil, to protect the rest of civilised Thera from being threatened via the wide bottleneck of Eldrinor.
Around 350 years ago, a great host of undead spilled out of every dark and horrible crevice in the east of the Barrens. This host was completely unheralded, but the armies of men were undeterred; in fact, they were confident that this would be yet another threat that they would crush underfoot. They were wrong.
The army of the undead grew impossibly quickly, there seeming to be no limit to the number of shambling corpses that could spew endlessly from the blackened bowels of the earth. Within weeks, the armies of men turned fully from supreme confidence to abject panic. There was deemed to be no possibility of defeating this seemingly infinite threat, and one order rang clearly across the land: "retreat". Every man and woman fled, and the undead flowed in an implacable tide of flesh and filth after them. There was no time to build walls to keep the undead at bay, but the world was surely doomed if humanity did nothing to stop this evil tide.
Thus, a now-legendary plan was made: an emergency force of volunteers was called upon to sacrifice their lives and save the world. The most selfless 20,000 humans and 500 wizards from across all of the armies stepped forward. Their task was simple: dig. They were to dig straight down into the ground as deeply as they could before they all were consumed by the encroaching undead. Magic and shovels blasted the earth while defenders above the trench held the line in what may have been the greatest display of military prowess, fortitude, and courage that any army has ever shown. A grand trench, now reverently named the Soldiers' Scar, was successfully dug all the way across the frontline. The trench stretched all the way from the steep cliffs overlooking the Panoptic Sea in an unbroken line to the frozen reaches of Wyrmsgrave where, for whatever reason, the undead dare not tread. These bravest of men and women completed their work with no hope of escape. They dug tirelessly, all the while knowing that it was both their and their adversaries' shared grave that they were preparing; that their eventual, inevitable reward for their labour would be a final blotting out of the pale sun by the rain of undead that would, sooner or later, tumble down and bury them.
The current soldiers of the Barrens hold these men and women in the highest possible regard. They are remembered as the most hallowed and venerated legends of the armies of the Barrens, and they are referred to in solemn whispers as The Trenchfallen.
Today, the Sisyphean task of the weary remnants of the great armies of the Barrens is to regularly bathe the entire length of the Soldiers' Scar in fire. This grim task is required because, to this day, the undead have never relented in surging forth in their endless multitudes, and if there were no intervention, the sheer physical mass of the accumulated corpses would pile so high that the Soldiers' Scar would be filled and a bridge would be made, of the horde, for the horde. This, then, is the destiny of the soldiers of the Barrens: to be Thera's most grim and essential of caretakers. Since the digging of the Soldiers' Scar, the armies have not left this post, and nor can they ever leave it, lest the world be eaten by the dead.