It was all Tywin and Walder's idea. At some point Walder contacted Tywin, then Tywin contacted Roose (I am guessing this was the letter we saw Tywin write after he found out Robb had gone to Riverrun for his grandfathers funeral and left Roose in charge at Harrenhal). Nobody else knew, other than Tywin, Walder and the Frey men, and Roose and the Bolton men.
Edmure = Alive. Blackfish = Alive. Blackfish must have something cool to do next week, because he never attended the wedding in the books, and there was no reason to write him into it unless he has something to do.
The Starks ensured guest right when they got to the twins, which is one of the oldest and most sacred traditions in the story. So even though Tywin was behind it, and the Boltons played a massive part, the Freys are mainly the shamed house for breaking that tradition. They'll be looked down upon even more than they already were now. By everyone, including the Lannisters.
Robb and Grey Wind both have their heads chopped off their bodies, they sew Grey Wind's head to Robb's body. As we saw earlier in the season with Catelyn's dad, the Tullys have sort of a viking-like funeral where they send their body out to sea. Catelyn's body is dumped in a river.
The write up of what fully happened in the book:
By using sellswords and knights at the feast disguised as musicians, and getting Robb's men too drunk to fight, Lord Walder Frey is able to coordinate a massacre with little loss to his own men. The two twin castles and the outlying camps are signaled when the band begins playing the "The Rains of Castamere" after the start of the bedding between Edmure and Roslin. Few of the northmen have a chance to react, and most are butchered on the spot.
The Frey and Bolton men turn on the Stark soldiers. Specially-rigged feast tents collapse and are set aflame during the slaughter. In the main hall, many of Robb's personal guard are killed by Freys, including Smalljon Umber, Dacey Mormont, and Ser Wendel Manderly, as well as his mother Catelyn. Roose Bolton personally kills Robb, who was previously shot by arrows, with a thrust to the heart.[1]
Though no definitive count is known, most of the northmen are killed while House Frey loses only approximately fifty men in the camps, as well as Ser Garse Goodbrook and Ser Tytos Frey, both of whom are killed by Sandor Clegane.[2] After the battle, the Freys cut off Robb's head along with that of Grey Wind, Robb's direwolf. In a mockery of Robb's relationship with his direwolf, the Freys sew Grey Wind's head onto Robb's decapitated body. Lady Catelyn Tully has her throat slit and is later thrown naked into the river in a mockery of House Tully funeral customs.
Robb's crown is kept by Ryman Frey.
