Here you can find few tips that might help you to unlock Merchant’s dream achievement. Warning! Guide contain spoilers!
Tips to Obtain Merchant’s Dream Achievement
That quest chain starts in the Green region (for act 2).
You need to go into the spider cave, and then successfully make it across the gap upon entering (Thuls can succeed at this without drawing btw).
After that, be sure to select ‘Explore’ rather than trying to get through the gap/crack in the wall.
This will get you attacked, but gives you a book quest item afterward, which turns in at the Happy Owl.
After that, you need to get to act 4b, and take the south route (to the colored runes).