On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough description of the Rock of Fertility side quest.
Rewards for completing the mission: 1,200 XP, silver pieces.
In the place marked on the image above, you will meet an arguing couple. The reason for the argument is that the man does not want to help his wife check a certain myth. The woman will tell you a story about a boulder imbued with great power - when a woman spends a night by it, she immediately becomes pregnant.
The woman wants to check it out and asks you to protect her during her travel - you'll come across some wolves along the way. Once you are there, you will find that the fertility stone is utter nonsense and the mission will end.