What was the scroll Arya found in Littlefinger’s chamber during episode 5 (Eastwatch) of Game of Thrones season 7? Analyzing the brief shot of the letter we saw, the scroll was written by ‘your faithful sister Sansa’ and referenced the death of King Robert Baratheon, somebody attempting to ‘steal his throne’ & also asked the recipient to ‘swear fealty to King Joffrey’. So this was most likely the letter Cersei Lannister forced Sansa to write to Robb Stark back in season 1, when she was held captive and Ned Stark was accused of being a traitor. Littlefinger was in the room as well:
Sansa was a child at that time and didn’t have much choice but to do what Cersei wanted her to do. However, the letter did not have the effect Cersei intended. Robb Stark called the Stark bannermen to war anyways as he saw right through it. But Arya & Sansa are unaware of the impact the letter might have had. And now the scroll is in Arya’s hand, who is already showing signs of suspicion towards her sister. And Littlefinger definitely wanted Arya to find that letter because if there’s tension between the Stark sisters, it’s only good news for him.