After defeating Rikiwulf and returning back to Fornburg in Assassin’s Creed Valhalla, you meet Sigurd. He introduces you to the Hidden Ones, and you get access to the Hidden Blade. After that quest, Kjotve sends his spies to gather intel about Eivor and his men. Eivor spots them and sets out to kill all of them.
Taking down the first spy is quite easy. His location is marked on the screen, and all you need to do is chase him and get closer to him. Once he is close enough, you just need to kill him, which will complete the first part of the quest. The second part requires you to locate the remaining spies in Fornburg.
To do so, you need to head back towards Fornburg after defeating the first spy. A quest marker will show up on the map, but once you get a bit closer to him, it just disappears, and it won’t give you the exact location of the spies.
To locate the spies, you need to use your raven. You can do so by entering Raven mode and exploring the area. The area where the spies are hiding will be highlighted in green. It is located away from Fornburg, towards the hills. You will see a camp and a bonfire surrounded by hills. That’s the place where the remaining spies are hiding.
Place a marker in the area with your raven to make reaching there easier. Once you reach the area, simply take out all the enemies. You will also find a note from Kjotve there inquiring his spies about the strength of Eivor and his men.
Once you are done with that, simply return to Fornburg and interact with Sigurd to complete the quest.