Background
The Atmorans (who would eventually become the Nords) were among the first organized Human settlers on Tamriel and had been emigrating from Atmora for many years prior. They called the land Mereth, in recognition of the vast number of Mer that lived there, and befriended the local Snow Elves. They built the great city of Saarthal, the first major Atmoran settlement in Skyrim as well as the first city of Men in the continent of Tamriel.
The Sack of Saarthal
One night, an invading force of Snow Elves descended upon Saarthal and slaughtered the inhabitants. The city itself was burned to the ground. These events became known as the Night of Tears. Originally, it was believed that the Snow Elves' motive was to exterminate the Atmorans as they realized the rate at which the human settlers' population was growing. If left unchecked, these settlers would become a great threat to Elvish civilization on Tamriel. Another theory, proposed by Imperial scholars, is that the Snow Elves' motives for attacking Saarthal was that they feared that the Atmorans' culture would surpass their own, and thus sought to drive the Men from their lands.
Some reports suggest that the initial attack on Saarthal was very focused, and did not appear to correlate to any locations that were established as points of defense or importance. Not only did the elves know the apparent layout of the city, but that their assault was based on a specific directive and perhaps a singular goal. It has been speculated that when Saarthal was originally built, the Atmorans found a source of great power located underneath its foundations, and attempted to keep it buried. The Mer learned of this and coveted the object for themselves.
A long-forgotten Nordic myth states that the Daedric Prince, Mehrunes Dagon, was responsible for the destruction of Saarthal. It is told that the Snow Elves had already hated the Atmorans, but Dagon gave the Elves grim dreams (most likely involving the Atmorans) that heightened their hatred of the Atmorans to the point in which they sacked Saarthal. Though the myth is rather eccentric and can't really be taken as entirely legitimate, there could be some truth to it.
The Return
Ysgramor and his sons, Yngol and Ylgar, survived and fled back to Atmora, but would later return with an army known as the Five Hundred Companions to reclaim Saarthal and lead a reprisal campaign against the Snow Elves. The ruined city was successfully retaken, and eventually rebuilt.
Ysgramor and his army hunted the Snow Elves and other Mer alike, ending the dominion of Mer over Skyrim and making possible the rise of Men on Tamriel, establishing the First Empire. Any remaining Snow Elves were brutally bound and burnt alive, or chained and enslaved. These series of events gained the name of The Return, and led to a lasting distrust and prejudice between Men and Mer.
Subject: The Night of Tears 
