On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Jotunheim side quest: The Giants of Fimbulwinter.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet two powerful Jotun twins: Kala and Vindr. The twins caused a snowstorm because a woman hurt their feelings.
After a brief exchange of words, the twins will grab their weapons, and you will have to fight two opponents simultaneously. It's best to focus on one to negate their numerical advantage and then finish off the surviving twin. After the battle, the sky will clear, and the mission will end.