On this page of our guide for Assassin's Creed Valhalla, you will find a walkthrough of The White Lady of Tamworth side mission.
Award for completion: 1200 XP.
On the place marked in the picture above, you will encounter a lamenting woman who was imprisoned in the tower by a soldier standing outside.
Fight him and the soldier will eventually give up and run away. When you talk to the woman through the door, you will find that it was just a show and they were just playing roles. The woman will ask you to become her guard instead.
Stand in the designated area in front of the stairs. The woman will start screaming again, and this will lure a true knight who will want to fight you. Beat him to finish the task.