The Red Wedding

Robb Stark broke a marriage oath to Walder Frey, and Walder Frey ÔÇö with Roose Bolton's knife and Tywin Lannister's blessing ÔÇö collected. Guest right is violated at the Twins and the King in the North, his mother, his wife and his army die between courses. The North does not forget it, and neither does anyone else.

The fuller tale

Robb Stark had needed a bridge, and the only bridge was the Twins, and the price of it was a marriage to one of Walder Frey's daughters. He gave his word and then broke it for a girl he had wronged and felt he owed, which was decent and which cost him everything.

Walder Frey accepted the substitute ÔÇö Robb's uncle Edmure would marry a Frey instead ÔÇö and invited the northern host to the wedding. Bread and salt were served. Under guest right, the oldest law the First Men brought into Westeros, a man who has eaten at your table cannot be harmed by you, and everyone in that hall had eaten.

The musicians in the gallery were crossbowmen. Robb's mother saw Roose Bolton's mail under his sleeve one heartbeat too late. The northern army was killed in the camps at the same time as its lords were killed in the hall. Robb was finished by Roose Bolton personally with a line about his father's regards, and his mother, who had a hostage's throat under her knife and no reason left to bargain, was killed last.

Why it is the worst thing in the book

Westeros has no shortage of massacres. This one is different in kind because of what it broke. Guest right is the one rule everybody keeps ÔÇö ironborn keep it, wildlings keep it, the Mountain's men keep it ÔÇö precisely because a country of armed men with no police needs one absolute rule about hospitality or it cannot function at all.

The Freys were rewarded with the riverlands and were despised by every single person in them, and no one would sit at their table afterwards. Tywin Lannister, who had blessed it, called it a bargain: two thousand dead at a wedding rather than ten thousand in the field. He was arithmetically right and it destroyed his family's standing anyway, because a realm that cannot trust a table cannot be governed by the man who broke one.

Elsewhere in the wiki

 See the world as it stood in 299 AC 

What became of it ÔÇö if an ending is written, it is here. Spoilers.

The chronicle records no ending here ÔÇö or none the maesters have set down.