In Assassin’s Creed Valhalla (ACV) you have to find and enter the well during the quest Mistress of the Iron Wood. This guide shows where to find the Well Location in Mistress of the Iron Wood quest. This is the first quest in Jotunheim (The Saga of the Snows Chapter).
Well Entrance Location
Once you arrive at the quest marker for the well location, you will notice there’s nothing there but some circular floor plate. From that circular floor plate, look south and you’ll see some stairs leading up to some rocks with carvings on them.
Read the carving on the rock at the top of the stairs.
Now turn around and the circular floor plate will have turned into a well. It’s Magic!
And that’s where you find the well during Mistress of the Iron Wood in Assassin’s Creed Valhalla (ACV).
The rest of the quest is just collecting some plants in the waterfall cave, you can ping them with 
/ 
to see them more easily. Then you need to escape the cave, always try to go higher in the cave until you see some light coming from the exit. Then bring back the roots to the quest giver in her house.
Thank you