If you noticed that the water from the central well of Tristram is with an orange color, your surely will have this quest.
After killing the Butcher, or either reaching dungeons level 2-3, back to town and talk with Pepin. He will talk about the water getting poisoned with some sort of substance he doesn't recognizes, and if don't do something quickly they all will die (since their water stocks are running out and people are getting sick).
Back track to the dungeons and find the Dark Passage (it will be at the dungeons level 2 or 3). Enter it and you will find yourself on something really close to a mining cave.
The only thing you need to fear is those "Clan demons", they are pretty tough to kill and if they come on a huge horde you can get killed. The other demons you will find here are those Fallen Ones that are lame.
Kill every demon on this cave and the water will magically become drinkable. Go back to town and speak with Pepin to tell him that the water was poisoned only 'cause of the demon presence close to it.
He will thank you and handle you an unique ring. (Or some random magic item, it depends on what replayability chose).