On this page of our guide for Assassin's Creed Valhalla you will find a detailed walkthrough of the Unholy Father mission.
Eivor joins Lifa who plans to kill the Walden priest, who is loyal to Wigmund. In this way, Eivor can get the information he needs to make it clear whether Lif is the traitor.
Rewards: 1700 XP.
Speak with Lif
Lif's plan is as follows: the warrior wants to sneak into Walden to eliminate the priest who is pouring tithes into Wigmund's treasury. During the conversation, you can ask Lif how he joined Soma's clan, as well as about his work in shipbuilding.
Find and assassinate the priest
While the rest of Soma's warriors distract the Saxons stationed in Walden, Eivor and Lif must find the priest and eliminate him. You can get to the right location quietly or challenge the enemy force directly.
If you want to do it quietly, do not enter through Walden's main gate. Instead, use the alternative entrance, on a wooden caravan.
You will find the priest in a brick church behind a wooden palisade with red flags. The main gate is closed so use the scaffold to get to the other side. Several more soldiers are stationed in front of the church.
Enter the building and eliminate the priest, then get out of Walden and talk to Lif to complete this mission. As a reward you will receive 1700 XP.