Stench

It breaks my heart to see what those foul greenskins have done to our ancient land. Not only have they built a shanty fort nearby where they practice all sorts of green magic but there's a veritable lake of filth not far from here, can't you smell it? The stench of that place, it's enough to drive even the most stout-hearted Dwarf back to the Marshes of Madness for a breath of fresh air! No matter I've come up with a plan to use their stench against them. We've made to attack that fort o' theirs and it's as if they catch our scent and are upon us before we can breach their defenses! Now if we were to cover ourselves with some of that filth we'd be able to approach the fort and be upon them before they realized! Me? Well I've got to stay here of course. You on the other hand could recover some of that muck and lead the Oathbearers waiting outside the fort to attack!

Travel east and enter Da Big Stink. Use the flask Vingaard gave you to collect some of the filth. Apply the filth to yourself and then report to the Oathbearer Veteran outside of Dammaz Skar on the Stonecleaver Trail.