The Long Night
A winter came that did not end. For a generation the sun did not rise, crops died, and out of the cold came the Others ÔÇö pale things with eyes like blue stars, raising the dead to walk behind them. The stories agree that mankind nearly ended and disagree about everything else, including how it stopped: a Last Hero with a dragonsteel blade, a battle for the dawn, and something that has been half-forgotten ever since.
The fuller tale
A winter came that did not end. The accounts agree on that and on almost nothing else. For a generation ÔÇö the number is always a generation, which is how oral history says a very long time ÔÇö the sun failed, the crops died, and famine did what famine does. Then, out of the farthest north, came the thing the stories have never had a settled name for: cold that moved with intent, and the dead getting up to walk behind it.
The reason to take the Long Night seriously is not any single source but the number of unrelated ones. Westeros remembers it. The Rhoynar sang of a winter that froze their river to its bed. Yi Ti dates a whole dynasty's collapse to a darkness in the east. Asshai has its own version, older and worse. Stories drift, but a story that turns up independently in a dozen languages usually happened to somebody.
How it ended is where the tellings scatter completely. The North has a Last Hero who went into the dead lands with twelve companions, a horse, a dog and a sword that froze and broke, and lost all of them before he found what he was looking for. Others tell of a battle for the dawn, or of a blade of dragonsteel, or of a woman with a monkey's tail, depending on who is singing. What every version keeps is the shape: everything was nearly lost, and something small and specific saved it.
The part everyone forgot
The most consequential thing about the Long Night is not that it happened but that it was allowed to become a bedtime story. Eight thousand years is long enough for a real enemy to turn into a figure of speech. By the time this chronicle reaches its final century, the men whose whole institutional purpose was to watch for a second one had been reduced to a thousand thieves and poachers on a wall, and the lords of the realm considered the whole business a quaint northern superstition and resented paying for it.
That is not stupidity. It is the ordinary working of memory over eighty centuries, and it is the closest thing this history has to a moral.
Elsewhere in the wiki
 See the world as it stood in 8,000 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.