On this page of the guide to Assassin's Creed Valhalla Wrath of the Druids, you will find a detailed walkthrough of the main story quest, The Northern Reach.
Although King Flann's army has been weakened, he has no intention at all of giving up on conquering Ulster.
Reward for completing the mission: 1,700 XP.
Speak to Flann
Flann asks Eivor for help in gaining the favor of the kings of Ulster, so you will have to complete several minor tasks for the rulers of the surrounding kingdoms.
Earn favor with Ulster's Kins
Approach a nearby Pigeon Coop and see what the Ulster Kings demands. To complete a quest The Northern Reach, you have to complete three tasks. Note that the tasks can be of different nature, but usually they're related to eliminating bandits or stealing certain items. If you manage to complete the assignment by observing all the guidelines, you will receive a small bonus.