The Nature of the Beast Walkthrough
When traveling to the left of the fortress, you will meet some trolls attacking merchants. Help them and find out about a mage living nearby. He should know how to end these scoundrels. At the same time, you will learn about the existence of Hodag — a monster that Amiri requires in her personal quest.
Go the nearby Lone House location, finish off some enemies and find Bartholomew in the upper right corner of the map. He will ask for help in the battle against the troll. Honor his request and select the first option to instantly go to the enemy. You can also have some preparation time and go to the place by yourself (left part of the map).
After your victory, talk to Bartholomew, go to his laboratory and head down. Examine the troll behind the bars, talk to the scientist and complete the quest. This will advance you through the Troll Trouble storyline. In the end, you can order Bartholomew to release the captive troll or leave him as is.