Well, did you get the quest from the chieftain's son in the Imp Village? "Find the alchemist"?
Kiya
Linearity goes like this:
You are on Citadel Prison, you free 2 imps and they show you the way over the lava.
After that you get a quest from the chieftain's son to find the "cause of the plague".
Speaking to imp Altoflix gives the solution - talk to the chieftain's son and report this. Then he will give you the new quest "find the alchemist".
Please, check your quest log.
http://www.kiya1.de/beyond/act2screens/Act2Chieftain'sson.jpgHere, this is a picture of the Chieftain's son. He is SW of the Chieftain's house, the only imp with a hat. and 2 houses NW of the chieftain's house is the hut of imp Altoflix. The one giving the solution for the plague prob.