Once you reach Act 2 of Baldur's Gate 3, you're presented with a curse that makes it impossible for people to travel. There are only a few safe spots in the entire land, but you're presented with a way to make the entire place safe. The thing is, it'll take a while.
How To Save The Shadow-Cursed Land
First off, you need to wake up Art Cullagh. Follow our guide to do that, then come back here. Once you've done that, he'll ask to see Halsin. Go back to your camp and tell him what Cullagh told you, then go straight back to Cullagh to progress the quest.
Halsin will tell you to meet him outside at the Lake Front, and once you do he'll open up a massive portal into another plane of existence, to rescue somebody called Thaniel. He's been trapped in the Shadowfell for over a century, and Halsin feels gravely responsible for it. He'll go inside the porta to attempt to rescue Thaniel, and it's up to you to protect the portal from three separate waves of shadows and shadow-infected people that will try and get to you.
However, once Halsin brings Thaniel back, he'll tell you that you need to find Thaniel's other half, his shadow self so that he can become whole. Travel to the new waypoint on your map to find the other half, who is a boy named Oliver. Oliver doesn't like being told what to do, and he heads into a portal that you need to follow him through.
Once you go through, you need to fight Oliver's Shadow Dog, Shadow Parents and damage a portal called a Nightdome to get to Oliver. Once you do all of this, combat will stop, and you'll be able to talk to Oliver in order to make him see reason. Oliver then departs to reunite with Thaniel, and you'll be instructed to defeat Ketheric Thorm in order to lift the shadow curse.