This page of the Assassin's Creed Valhalla game guide contains a detailed overview of the mission War in the North.
The news of Eivor's actions in Jorvik reached Halfdan, so he decides to call him for talks.
Rewards: 3400 XP.
Go to Anlaf's Lookout Tower
After reaching the site, you will notice signs of battle. Climb up the hill to meet the warriors of the Halfdan clan and talk to Faravid. The man will tell you that they are fighting hard with the Picts.
Defend Faravid and his men
After a while, more hostile warriors will arrive from the hill. Repel this attack. After the fight, talk to Faravid and watch a short cut-scene showing Halfdan's return. Jarl will tell you he's willing to talk, but not here and not under these circumstances.
Meet Halfdan Ragnarsson
Go to Donecaestre for a meeting with the yarl, you will find him in the main fortress. Ricsige will also be there – may remember him from your visit to Jorvik. From a short conversation with the yarl, you will learn that Halfdan apparently suspects one of his people, Faravid, of double-crossing and betrayal, but does not want to talk about it in front of witnesses.
Speak to Halfdan
If you want, you can take part in many activities related to the feast (dancing, fistfighting) or go to a private conversation with Halfdan right away. Jarl will tell you about the importance of friendship. Halfdan believes that Faravid is conspiring behind his back, but unfortunately there is no evidence of this. He offers Eivor a deal – if he proves the betrayal of one of the jarl's most trusted men, he will find loyal allies in the Ragnarsson clan.
After the conversation, the mission will end and you will receive 3400 XP.