Remember Walder Frey? The head of House Frey, the Lord of the Crossing and a former vassal family of House Tully of the Riverlands? In addition to being a lecherous old creep (with a 15-year-old wife), he also betrays Arya's mother and brother, Catelyn and Robb, when he orchestrates the infamous Red Wedding massacre in season three.
To get her revenge for his devastating betrayal, Arya disguises herself as a servant in House Frey in the season six finale. She proceeds to serve Frey a pie that he realises too late has been baked out of the remains of his two sons. As he reels back in horror, Arya reveals her true face and slices his throat, ensuring that the last thing he sees is the smiling face of a Stark.
Adding insult to injury, Arya then wears Walder's face in the season seven premiere so that she can poison his entire family. Now that's how you do revenge, people.