It was the letter that Cersei manipulated Sansa into sending to Winterfell in Season 1, telling Robb about Ned’s “treason” and asking him to come bend the knee. (WotW has a pretty clear screencap here.) Petyr’s endgame (on the show) has always been about accumulating as much power and influence as he can, and he stands the best chance of doing so if the Stark sisters are divided. Unlike the books, show!Petyr’s main function is just to stir up as much shit as he can, then capitalize on the fallout with little to no advance planning.Talking about endgames for show!Petyr is a pretty pointless exercise now.