Start by talking to Denulth, and he will explain the failure of their raid. He willalso explain Godric, Dunstan's son, was kidnapped. Ask if you can help. Head towards Tenzig's hut. If you don't have climbing boots from him, get them.
Head out north of his hut and follow the path below.
Dad & Trollheim
You will come across an arena with a large troll named Dad. Talk to him and he will challenge you to a fight. As he is almost dead, he will ask to be spared.
Continue on the path to the north and head into the cave. Go through the cave and out the other side, then follow the below path. If you have 43 agility, it is highly recommended to take the cyan path. If you take the read path all the way around, you will be attacked by stone thrower trolls.
Head into the Troll Stronghold.
Upon entering the Stronghold, head south. Look for a door on the west side of the hall, and then go through and kill a Troll General. It will drop a prison key. Now head back into the hallway and go to the far north.
Go down the staircase to the far north, then unlock the nearby Prison door. If you are 30 thieving, pickpock Twig and Berry for the keys. If you can't, you will have to kill them. It is recommended to free Eadgar as well.
They will return back home. Go to Dunstan the smith, and he will reward you.