The Red Wedding refers to the massacre that took place during the wedding of Edmure Tully and Roslin Frey, in which Robb Stark, his wife Lady Talisa, his mother Catelyn and the majority of his 3,000+ bannermen were all slaughtered by House Frey.
Lord Walder Frey, otherwise known as Black Walder, Lord of House Frey, openly rebelled against King Joffrey and the Iron Throne and declared his support for Robb Stark, the King in the North. However, after King Robb broke his vow to marry one of the Frey daughters, Walder Frey began secretly corresponding with Lord Tywin Lannister to negotiate the terms of a Frey allegiance to House Lannister and the Iron Throne. Lord Roose Bolton was also a key figure involved in the planning of the Red Wedding.
After beheading Lord Rickon Karstark and losing all the Karstark bannermen, Robb, desperate for military reinforcements, agreed to marry his uncle, Edmure Tully to a Frey daughter in order to secure the allegiance of House Frey, as well as the strategic position of the crossing, which is controlled by House Frey.
Upon arrival, Robb’s company invokes the guest right after breaking bread and salt with Walder Frey. Under this sacred covenant, neither guest or host may bring harm to the other for the duration of the guest’s stay under the host’s roof. Despite this, Walder Frey breaks this sacred law of hospitality, something is only bold enough to do with the backing of Tywin Lannister.
After the Rains of Castamere begin to play, the massacre begins as Lady Talisa and her unborn baby are first slaughtered. Arrows then rain down upon King Robb and his mother Catelyn. After begging the mercy of Walder Frey to let her son live, Roose Bolton is the one to put a dagger through the belly of Robb as he tells him “The Lannisters send their regards.” Before Catelyn is murdered, she appears to be frozen with shock for several seconds. Robb’s direwolf, Grey Wolf, is also murdered, along with the majority of Robb’s army and bannermen.