On this page of the Assassin's Creed Valhalla Crossover Stories guide you can find a walkthrough for Night and Day story quest. Eivor and Kassandra have to explore a mysterious cave.
Starting the mission
Kassanda will be waiting for you north from Tobhta village, by the main road. . .
Hop on your mount and follow Kassandra.
Along the way, you'll be a witness to a conversation between both heroes. They will find a map to the cave you need to visit.
Once again, hop on your mount and make the journey. Kassandra rides independently of you. You can lose sight of her if you are riding too slow or too fast.
The end of your route is a cave that lies at the edge of a body of water. You will enter it without diving in, only getting your clothes wet.
Inside you will see empty walls that you can't do anything with. However, when Kassandra comes, she will make some of the stones disappear and your eyes will see a monolith, into which you must put the shards. This is the path to an artifact that will save the island from going mad.
The final part of the quest is a battle against numerous opponents. Eivor will notice a strange language used by the enemies. Focus on battling the opponents and clear them out before they harm you or Kassandra.
There will be an extended conversation between the characters during this scene. Regardless of what dialgoue options you choose, the outcome will be the same.
To continue, proceed to the Counting Sheep walkthrough, found on a separate page of this guide.