On this page of guide to Assassin's Creed Valhalla Wrath of the Druids contains a detailed walkthrough of the main story quest, Gathering Strength.
Eivor helps the surrounding kings, who send men and supplies to Flann as a token of their gratitude. Eivor is to meet him at Konckfree, where they will devise a plan of attack.
Reward for completing the mission: 1,700 XP.
Speak to Flann in Knockfree
Go to the location indicated on the map and talk to Flann. The Arch-King of Ireland will be grateful to you for helping the kings of Meath, who have commanded their soldiers to bolster the ranks of his army. The Gathering Strength quest will end, and you will receive 1,700 XP as a reward.