On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for The Great Scattered Army quest.
Eivor sets off to meet the Summer Army commander, with whom he wants to form an alliance. Troops of his army were recently seen near Grantebridge.
Rewards: 3 400 XP.
Find and speak to the Summer Army's leaders
You will find the required NPC right next to the synchronization point. A man named Magni stands at one of the viewpoints and looks towards Grantebridge. When you talk to him you will hear a story about how his unit was decimated during the fight against the Saxons.
Find and reach Soma's longship
After talking to Magni, you need to find Soma's ship, which is located somewhere in the marshes of Grantebridgescire.
Reach the Middelturn wetlands and get ready to fight, as there is a fierce battle going on in this treacherous area. Use the Raid option and clear the area of enemy soldiers.
Find and speak to Soma
After the fight is over, Eivor must find Soma and then talk to her. Her location is not marked on the map so you have to look around the swamps.
For reference, we have marked the location of Soma in the picture above. You will find her in Middelturn where she helps with inventory management and takes care of wounded warriors.
From the conversation with Soma you will find out that she managed to gather several soldiers, but the three most trusted advisors are still missing: Birna, Galinn and Lif. When you agree to find Soma's people, the mission will end and you will receive 3 400 XP as a reward.