This mission sees you teaming up with Idril again, this time to poison a bunch of grog.
Your secondary objective is not be detected, so let Idril poison the first barrel for you (not that it counts toward your total), and start sneaking. Just remember that speed is handy here- once you've poisoned a barrel, orcs will start drinking from it, but that means they'll be moving toward the poisoned barrels, so don't linger too long. Likewise, after a while orcs will start to die of poison, which also causes a bit of ruckus.
The first easy target is just a few feet dead ahead.
Now double back to the start and bear right. The rightmost objective marker is another vulnerable barrel hidden in a hut. Enter through the right-hand entrance to remain hidden.
Just past the hut is a tower with nobody on it. Jump up, then sneak over the ropes. You'll see two objectives at the base of the tower you're moving towards- you want to poison the far, out-of-sight one.
Turn around and you'll see an unguarded barrel up some ruined stairs.
At this point, orcs are probably starting to drop, and the ones still standing are going to investigate. Wait for a near barrel to be unguarded then get your final objective (the one near the stone tower is probably easiest- it only has one guard anyway).
This triggers the entrance of the local captain. You and Idril will get to work together to defeat him.
When he dies, Idril will discover the artifact she was hunting. This actually counts as a Lost Artifact for your collectibles- you can't complete the Finished Tales achievement or get the Prospector skill upgrade without doing this mission.
The end of the cutscene ends the mission.