On this page of the Assassin's Creed Valhalla guide, you will find a detailed description of how to complete the mission Let Them Eat Ashes.
Before hitting the fortresses of Portcestre, Eivor must weaken Fulke's strength, so he decides to burn the grain stock at Gildefort.
Rewards: 1,700 XP.
Speak to Ubba
Ubba is waiting for you at St. Lewinna's Church. The man is convinced that they have an awful lot of guards for a farm settlement like this. After a brief discussion of the action plan it is time to get down to work.
Burn the grain silos
The village is indeed very well guarded so you have to prepare for a lot of resistance if your presence is detected. The main goal of the mission is to burn grain silos, which are the constructions characterized by red roofs. When all the silos burn down, you will receive 1 700 XP as a reward.