The Red Wedding was one of the major climaxes in A Storm of Swords.
It came about because Robb Stark, who was betrothed to a female of the proud Frey clan, married Jeyne Westerling, of a lesser house. Robb still needed the Freys, bannermen to Riverrun who were his primary support, to help him win the War, and so tried to treaty with them and win back their loyalty. At Walder Frey's insistence, he and his fleet went to the Twins, where Edmure Tully, Robb's uncle, married a Frey woman. Catelyn Stark, Robb's mother, believed that they were safe after partaking in food and drink at the Frey's table, but they soon discovered that the Freys had brought them there only to kill them. The Frey men slaughtered the small Stark host and killed Robb and Catelyn as well. They cut off Robb's head and replaced it with the head of Grey Wind, his direwolf, and threw Catelyn Stark into the river after cutting her throat.
This event became known as a cursed happening with other major characters, and caused the Freys much grief following it.