On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough description of the Eivor the Sheepdog side quest.
Rewards: 1,200 XP.
In the place marked in the image above, you will meet a man who has lost a sheep named Fenton somewhere.
Head into the nearby forest and you will find Fenton cornered by a pack of wolves. Eliminate the threat, then whistle at the sheep and lead it to its owner. The man will thank you for your efforts and the mission will end.