Bastard Letter and Pink Letter are fan-given names for a letter purportedly written by Ramsay Bolton.
A Dance with Dragons
The letter is delivered to Lord Commander Jon Snow by the steward Clydas at Castle Black. It is addressed to "Bastard" and is sealed with pink sealing wax, which is used by House Bolton. The letter states:
Your false king's friends are dead. Their heads upon the walls of Winterfell. Come see them, bastard. Your false king lied, and so did you. You told the world you burned the King-Beyond-the-Wall. Instead you sent him to Winterfell to steal my bride from me.
I will have my bride back. If you want Mance Rayder back, come and get him. I have him in a cage for all the north to see, proof of your lies. The cage is cold, but I have made him a warm cloak from the skins of the six whores who came with him to Winterfell.
I want my bride back. I want the false king's queen. I want his daughter and his red witch. I want this wildling princess. I want his little prince, the wildling babe. And I want my Reek. Send them to me, bastard, and I will not trouble you or your black crows. Keep them from me, and I will cut out your bastard's heart and eat it.
Jon reads the letter to the illiterate Tormund, who is skeptical of its contents. Convinced some of the letter is accurate, Jon announces in the Shieldhall he will march to Winterfell, which leads many free folk to volunteer to accompany him. After leaving the hall, however, Jon is stabbed in the mutiny at Castle Black.