This quest begins immediately after Glory Regained.
From Grantebridge, head directly south to a small church holding named Walden. If you’ve activated the synchronization point southeast of Meldeburne, you can use that to close the distance somewhat. Otherwise, you’ll need to cross the river south of Grantebridge first. You’ll find Walden waiting with a few warriors southwest of Walden. Speak with him to learn of his plan, and to ask a few questions regarding how he came to join Soma’s clan, and what it’s like to be the master shipbuilder for her. After the conversation, Lif will advise that his warriors distract the guards at the main gate, while you and Lif find the priest.
When you’re ready, head directly east from where you met up with Lif. He will follow as you go, but you’ll want to avoid the archers on the walls. If need be, take them out with arrows to prevent them from raising the alarm. Your main objective here is to reach the church, which you can approach from the southeast or northeast to avoid most of the guards. When you reach the walls, use Odin’s Vision to highlight any guards in the vicinity. Don’t be fooled by those who carry keys to the church; you don’t actually need this key for anything special. You won’t be able to enter the church area by climbing the high wooden stakes. Instead, you’ll find a large tree at the corner of the eastern wall which you can climb to enter the main churchyard.
There’s a good chance that Lif will blow your cover here, but most of the extra dialogue will have occurred at this point anyway. There are multiple methods of entry into the church, but the easiest is by breaking a window on the ground floor and jumping through it. You’ll find the priest at the southern end of the church, in front of a Book of Knowledge. Assassinate the priest and then collect the book to learn the Incendiary Powder Trap ability. Take some time to loot the minor treasure in the church, then flee the area with Lif when you’re ready. You can leave the way you came in to avoid guards, or you can storm out by force through the front gates for some extra XP and loot, including a Carbon Ingot looted from a Yeoman.
When you leave Walden with Lif, he’ll run back to the camp where you first met. Speak with him there to complete the quest.