Silently as a cat, Arya Stark disguised herself and appeared in Walder Frey's hall with a tasty meat pie for the old man to eat for dinner. So what did Arya feed Walder Frey on Game of Thrones? Let's just say Arya has picked up a recipe or two, along with her mad assassin skills.
We set the scene with a huge feast at Riverrun, in celebration of Walder Frey retaking it from the Blackfish with the help of the Lannister armies. In return, of course, Frey pledges Riverrun to the Iron Throne and declares a new alliance between House Frey and House Lannister. (They even coin themselves a new catchphrase: "The Freys and the Lannisters send their regards.") During the celebration, an attractive young woman lingers on Jaime while serving him wine. We later see the same woman serving Walder meat pies in the empty hall.
He notices that she isn't a lady of his court — "too pretty" he remarks, before landing one of his token creepy butt-smacks. He assumes she's from the Lannister camp and has simply been sent up with his food. Walder demands to know where his sons are. He's eating in an empty hall at night, and they were supposed to arrive at midday. The woman keeps repeating that "they're here," and Walder is understandably confused. Until, that is, the woman points out the meat pie sitting on his plate.
He pulls back the crust and sees that his sons have been baked into the pie, which is literally a scene straight out of Titus Andronicus.
"They weren't easy to carve," the woman flippantly remarks, before — obviously — pulling off her face to reveal that she is, in fact, Arya Stark.
"My name is Arya Stark," she calmly says. "I want you to know that. The last thing you're ever going to see is a Stark smiling down at you as you die."
She then slits his throat easily and patiently holds his head back, smiling all the while, for him to bleed out as efficiently as possible. Jaime seems to have escaped the Great Arya Butchering of 2k16, since we see him arrive back at King's Landing just before Cersei is crowned queen. But I'm betting nothing can kill Arya's buzz off of avenging the Red Wedding. Well played.