- Talk to Ephrahim to get the mission
- Walk to Skook's Corner
- Ask landowner Tallin about the forest cave
In the last mission, after your conversation with Ephrahim, you are told someone dealt with that cave in the past. He tells you what he remembers of it, and sends you to Skook's Corner to talk with a landowner named Tallin.
You walk to Skook's Corner and find him in the first mansion just at the entrance of the town.
When you talk with Tallin, he explains to you that something was done in the past with the cave, but he complaints it wasn't enough. He explains that it is a long story, and that now is the time to end it.
He will give you the cave's key and will explain what to do now, but that's in the next mission: Heavy Burden.
If you have Skook's Corner monolith active you can use it to make the trip to the town a little shorter. Anyway it's not so far.