Complete all the available Underground side-quests in both the North and the South
This is story-related and cannot be missed.

These are side missions are required as you go through the main storyline. You'll see a message telling you to find medicine. Simply go to the priest in the main town and he'll give you some documents to deliver in exchange for more medicine. Simply go to the house marked on your map, kill the enemies outside, and then go in to deliver the papers.