Blades of Exile if the forth of the Exile Trilogy by Spiderweb Software (Jeff Vogel).
It's an RPG where you aren't from Exile (in the first two of the three scenarios anyway) unlike the previous three games.
In the first scenario you must find what is poisoning the water supply.
In the second one there are rebels whom you must either stop or assist. (Your choice.)
In the third you must travel somewhere in a short period of time.
It's the first Exile with a scenario editor.
There have been several scenario contests, the first one run by Spiderweb Software and the others (to date, there have been three total plus, I belive a "short scenario" contest) by fans.
See Also: Exile: Escape From the Pit, Avernum, Riddle of the Spheres