Pure white oath-cloth given by Varré. The final trial to be anointed a Knight of Mohg, Lord of Blood. “Soak the cloth with a maiden’s blood… The blood of anyone’s maiden will do. You are maidenless, after all.”
Where to Find the Lord of Blood’s Favor
White-Faced Varré gives it to you when you accept his second quest, by choosing to be appointed as a knight of Luminary Mohg, the Lord of Blood.