Mouth of Miasma

Now take this rotweed extract and find where some of those chuffin' brutes are drinkin' the fouled ale. Pour it in, and it'll make the booze a deadly poison to those greenskins who dare t' drink it for at least a full day! Oh, they'll be gettin' a bit more kick than they bargained for out o' that beer!

Take the Rotweed Extract, and then travel south. Find a barrel of the Mushroomed Ale, and pour the extract into it by right clicking on it in your quest inventory while the barrel is selected. Then give the barrel a good swirl to mix the Rotweed in. Return to Thorgrund Foreghand at Dunhilda's Lads when you've succeeded.