On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for The Pardoner's Tale side quest.
Rewards: 1,200 XP.
In the place marked on the image above, you will meet an elderly habit-wearing man. Talk to him and listen to his story.
Take the hint the man gave you and go to the big oak tree that grows right next to you. Examine the food and the bodies of the three men - one was strangled and the other two were poisoned.
Go around the tree and examine the urn and you will find some silver inside. At this point, a short cut-scene will activate, in which Eivor will analyze the traces of the crime, and the quest will end.