On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the Will o' The Wisp side quest.
Rewards: 1,200 XP.
In the place marked in the image above, you will hear a mysterious voice announcing that you have entered a land that's under a terrible curse. Among other things, you will hear that you can save a life by making an offering of silver.
Behind one of the ruined walls, you will find one of the men involved in this play. From your conversation with him, you will learn that they scare travelers to extort valuables from them. During the conversation, you can decide what will happen to the men (leave them or knock them all out) and also give them a few pennies.