I've reached the Overseer's Chamber, lit the area up, defeated its inhabitant and ate his eye. This opened no new areas to explore, however. Now what?
Defeating him didn't complete any quests I had, while I still have quests open such as "The Shimmering Deep" or "A Cure for Corruption" that apparently don't find an answer here and now.
Best Answer
The Shimmering Deep is an optional sidequest that involves finding a piece of ember, so it would not be completed by finishing the boss, and the ember may very well be further down below Torchlight.
As far as "A Cure For Corruption" goes, you need to find the Overseer's book after defeating him to complete the quest. I found some details on the Runic forums below on the location of the book.
Taken from: http://forums.runicgames.com/viewtopic.php?f=5&t=2242