After you executed Rickard Karstark, half of your army abandoned you. Since you were at war with the Lannisters, you had to find a way to replace your losses. The first idea that came to your mind was regaining the support of Lord Walder Frey, whom withdrew from aiding you in battle ever since you broke the promise to marry his daughter.

Surprisingly, Walder agreed to help you defeat the Lannisters under certain conditions. He demanded you give him Harrenhall, as well as the marriage between Edmure Tully and his daughter Roslin. Seeing this as a great opportunity, you pressured your uncle, Edmure Tully, to accept.

Later that month, you arrived at the wedding with your entourage. Lord Frey showed you hospitality, but didn't waste a chance to mock you over the oath you broke. "My honored guests, be welcome within my walls and at my table. I extend to you my hospitality and protection in the light of the Seven."

Once the newlyweds retired for the bedding, the musicians start playing a song you were familiar with, The Rains of Castamere, which was an infamous song dedicated to house Lannister in general. Upon seeing that the halls have been sealed, you knew something was off.

Walder Frey eventually holds a hand up to cue the musicians to cease playing, and starts speaking with an evil smirk on his face. "Your Grace, I feel I've been remiss in my duties. I've given you meat and wine and music, but I haven’t shown you the hospitality you deserve. My King has married and I owe my new Queen a wedding gift."