On this page of the Assassin's Creed Valhalla game guide, you will find a detailed description of the Scorched Earth mission.
Before hitting the fortresses of Portcestre, Eivor must weaken Fulke's forces, so she decides to destroy the supplies stored in Crawleah.
Rewards: 1,700 XP.
Speak to Soma
Soma is waiting for you skulking near the Crawleah settlement. You may decide to openly storm the settlement or follow Soma's advice and take advantage of the fiery arrows. If you decide to fight openly, Soma and her people will naturally help you in the fight.
Burn down the Crawleah
Your progress in the mission is made by burning houses and carts with supplies. While performing this task, you will of course be disturbed by the guards. When the progress bar is 100% full, the mission will end and you will receive 1 700 XP as a reward.