The First Men cross the Arm of Dorne
Men came east to west over a land bridge, carrying bronze blades and leather shields, and began cutting down the weirwoods as they went. The children answered with what the songs call the Hammer of the Waters, shattering the Arm and drowning the bridge ÔÇö the first time in these histories that someone breaks the world to win a war, and not the last.
The fuller tale
The First Men came out of Essos on foot, across a land bridge where the Stepstones and the broken Arm of Dorne are now, and they came for the same reason people have always moved: there was better land on the other side. They brought bronze, leather, horses and dogs, and they brought the habit of clearing ground ÔÇö which is what actually started the war. To a farmer a forest is an obstacle. To the people already living in it, it was everything.
The killing was not a campaign; it was two thousand years of a frontier moving slowly north and west, with atrocities on both sides that the songs are very selective about. Men cut down weirwoods, which to the children was not deforestation but murder. The children answered from cover, at night, with poisoned arrows and worse, which is why the First Men's own stories are full of things in the dark that hate you.
Then the Arm broke. The songs say the greenseers gathered on Cape Wrath and called down the Hammer of the Waters, and the sea came through and the bridge was gone, and every man already in Westeros was in Westeros for good. It is the first time in this history that somebody breaks the world to win an argument, and it did not even work: enough men had crossed already.
What the First Men left behind
They are the reason half the map is named the way it is. The runes on the barrows, the ringforts, the ancient dyke-and-ditch works that later lords built castles on top of ÔÇö all theirs. So is the oldest law in Westeros, guest right, which they carried in with them and which is still the only rule in this story that everyone claims to keep and everyone is horrified to see broken.
Their blood is thickest north of the Neck and in the mountains of the Vale, thinner in the Reach and the riverlands where the Andals settled hardest, and mixed everywhere. A southron lord who calls himself of the blood of the First Men is usually making a point about how old his line is. A northern lord saying the same thing is simply stating where he is from.
Elsewhere in the wiki
 See the world as it stood in 12,000 BC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.