This quest begins immediately after The Man Behind the Man. After the assault on Rouecistre Fortress, Cynebert has agreed to provide you with information on the location of Fulke. You’ll need to meet with Basim back at Saint Paul’s Priory first, located northwest of Rouecistre Fortress.
Speak with Basim at Saint Paul’s Priory
When you arrive at the priory Eivor will note that it seems too quiet. You’ll find Basim on the eastern side of the priory. Speak with him and then walk to the Abbot’s chambers. When you do, a cutscene will begin. After the scene, you’ll need to escape the chambers from the incoming ambush. In truth, the ambush isn’t much of a danger (you’ll only need to fight a handful of guards), but if you really want to leave without bloodshed you’ll find an exit up the ladders within the chamber. Destroy the boxes and then move the barricade to leave. You can then kill the guards on your own terms if you wish, or just run away from the priory.
When prompted, speak with Basim about what to do next. A brief exchange between Eivor and Basim will result in the decision to investigate Canterbury for any trace of Sigurd, and with the intention of finding Fulke there, too. You’ll find Canterbury far to the southeast in Cent. It’s not recommended that you fast travel there, as you’ll experience some interesting dialogue with Basim along the way.
Infiltrate Canterbury Cathedral
When you arrive at Canterbury, Basim will dismount and suggest that you infiltrate the cathedral together. You’ll find it in the southern part of the town. For ease of access, approach the town from the south and climb the walls. You’ll then find the cathedral directly in front of you. Be sure to put your cloak on to avoid unwanted attention from guards, but they’ll still detect you if you get too close, so try to avoid them. As you approach the cathedral from the south, you’ll find a door which you can force open with the help of Basim.
Once inside, you’ll need to defeat a group of guards. This will include some elite units such as Skirmishers, but nothing you can’t handle, especially with Basim at your side. When all the guards are dead, turn your attention eastward toward the key symbol (use Odin’s Sight if you haven’t already to highlight it). Walk in the direction of the key and head down the steps beneath the effigy to Fulke’s sanctum.
Explore Fulke’s Sanctum
At the bottom of the steps, you’ll see that a Housecarl holds the key. You’ll need to defeat him before you can proceed.
The Housecarl is swift and can inflict some high damage attacks, so you’ll need to be ready to avoid those with a dodge or roll, or parry / block them. His special attack will be indicated by a red rune aura, which you won’t be able to parry or block, so you’ll need to dodge this. Ultimately the fight will be easy since you can essentially kite the Housecarl while Basim inflicts damage. Once you have the key, enter the sanctum by unlocking the door.
When you enter Fulke’s Sanctum, you can interact with numerous objects in the room to learn more about it, and a little more about Fulke, especially from two notes at the eastern side of the room. Take some time to read these, and then proceed through the sanctum to the southwest. When you enter this last room a cutscene will begin, after which the quest will be complete. Be sure to enter the room again at the end to read one last note detailing the experiments on Sigurd.