The Stygian Lurker - Act 4 Side Quest

Start Location: The River Styx
Quest Location: Stygian Marsh
Objective: Defeat Shade Feaster in the Stygian Marsh
Reward: Permanent Increase to Strength, Dexterity and Intelligence

The Stygian Lurker Side Quest begins by speaking with Copreus, an NPC found in the small town of The River Styx. You can take the Portal to Styx and find this NPC a little bit to the east (pictured above). This quest is one of the few quests in the game that will give you a permanent reward for completing it. You'll receive an increase to your Strength, Intelligence and Dexterity stats on each difficulty.

For this quest you'll need to find and defeat the Shade Feaster (pictured below) in the Stygian Marsh. You'll come across this area shortly after defeating Charon and crossing the bridge. You'll find the Shade Feaster in the northern most potion of this marsh, as shown in my map screenshot below. He's an easy fight on all 3 of the difficulties, assuming you have the appropriate resistances. The quest will automatically complete once he is dead - you won't have to return to Styx to turn it in.

Note: Every time you log back into the game after completing this quest you'll see a notice saying that the quest completed again. It'll also say that you've been given the reward again but this isn't true - it's been tested and confirmed to be just a bug.