Salem, the sequence is this:
1. Get "cause of plague" quest from the chieftain's son
2. Talk to imp Altoflix (2 houses NW of the son's location)
3. Answer the DK question and return to the son to report
4. Get quest to find the alchemist from the son. Alchemist's cave is in the fire area and he will be in there now. Before you didn't solve steps 1-3, this cave will be empty.