On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for A Blood Hymn of Edmund side quest.
Rewards: 1,200 XP.
In the place marked in the picture above, you will meet a nun named Aelfwyn inside the ruined church. The woman prays fiercely to God to not forgive the invaders. Instead, she asks to send them straight to hell.
When you talk to her, you will discover that the bodies lying at the altar were her doing. The woman will attack – you have to defeat her. Her death will end the quest.