Thestios told you he would provide you a reward if you wiped out all Balaur in the Sulfur Tree Nest area. Go to the Sulfur Tree Nest and do your work.
Step 1. Wipe out all of the Balaur in the Sulfur Tree NestA hidden chamber that can only be reached from within the Sulfur Fortress. (x63).
Step 2. Talk to ThestiosAn overseer guarding the Sulfur Tree Nest deep in the Sulfur Fortress in Reshanta..