On this page of the guide to Assassin's Creed Valhalla you will find a walkthrough of the side mission called The Twit Saga, Part 1.
Award for completion: 1200 XP.
In the place marked in the picture above, you will find two men arguing that none of them took the torch although they were supposed to burn down a nearby cottage to practice Raiding.
Fortunately, Eivor always has a torch on hand, so use it and set fire to the nearby building. After it goes up in flames, talk to the brothers and they will invite you inside to help them collect treasures, including their mother's precious axe.
The brothers had a key to the door, but unfortunately, they left it inside so you have to find an alternative entrance. At the back of the cottage, you will notice a barricaded passage. Remove the barrier to get inside. After entering the house you will be able to get additional loot and open two smaller chests with valuable supplies.
The quest item that the brothers are so worried about has been well hidden. The warriors will give you hints on where to find it. The first clue will be beer barrels.
If you don't find the axe in the barrels, the brothers will send you to a small room with bags. You will find them behind a makeshift wooden door that can be destroyed. Unfortunately, you will not find any weapons there.
After a while, one of the brothers will start screaming that they've found the axe. It seems that they had it somewhere outside all the time. Return to them to complete the quest.