The Great Scattered Army is the first quest of the Grantebridge arc in Assassin's Creed Valhalla, and likely the next main story quest you'll do after finishing either Heavy is the Head or Hunted. It tasks you with finding the Summer Army, which has been spread out across Grantebridge after its leader, Soma, lost her keep to a Saxon named Wigmund.
If you're stumped about where to find the Summer Army, this The Great Scattered Army guide from our complete Assassin's Creed Valhalla walkthrough will help you out.
Assassin's Creed Valhalla The Great Scattered Army Guide
Ideally you'll start this quest right after finishing Hunted — if you're coming from somewhere else, I recommend consulting the Hunted guide linked above to make sure you've done everything you need to do in Ledecestre and ensured that Zealots don't hunt you for literally the entire remainder of the game.
Once you've finished Hunted, board your longboat at Ravensthorpe and head southwest. To actually trigger this quest you'll need to pledge to Grantebridge by going to Randvi and selecting it from the Alliance Map. Once you've done that, you'll have a quest marker highlighting your destination and can leave for Grantebridge.
When you first arrive in Grantebridge you will see a large bridge over the river. If you are facing the bridge from the direction you came in, you will need to alight on the right-hand side of the bank in order to go the correct area. This shore will be the one to your northwest on the minimap. Use Raven's sight to pinpoint the exact location of the ruins you need to go to — they will glow bright green when you fly over them, so you can't miss them. While you're flying as Synin you can also place a custom marker in the ruins, which will cause the location to show up on your minimap.
These ruins are where the Summer Army is currently camped. It is worth noting that a synchronization point is right next to the ruins and is a downright essential fast travel waypoint for the next few missions, so climb that and synchronize with the area first. Once you've done that, head into the ruins directly beneath you. Using Odin's sight should highlight the person you need to speak to in order to advance the questline. They are to the west of the synchronization viewpoint, but the person you need to talk to is directly right of the leap of faith haybale below it. The easiest course of action is to simply jump down and exit the haybale to the right, at which point you will see him.
Head over and speak to him after landing in the haybale. His name is Magni and he will explain that the Summer Army recently lost Grantebridge to the Order of the Ancients — your next objective will be to find Soma in The Song of Soma.