Grassblade was aghast to hear how the life energies of his Green Hood comrades who fell when Deathfist Citadel was retaken by the Deathfist forces, have somehow been stopped from rejoining the life flow. Being an accomplished burglar, he has volunteered to not only show me how to gain access to Scourge Keep, but to also help me solve this mystery.
Speak with Grassblade once inside the Keep ( -7, -17, -125 ) /waypoint -7.05, -17.03, -124.93.
He is 180° around the moat from where you came in (of course!)
Tip: Don't kill any (or as few as possible) Orcs prior to getting to Grassblade. Right by Grassblade is a skeleton on a pike. Clicking that will start a timed quest to kill 100 Orcs in 10 minutes.
Tip: While still outside the Keep, get the item ("a blood sap-tipped arrowhead") off the guy pinned to the tree with arrows (I assume it's click-able prior to entering the Keep itself, I went and got it after I'd be inside for a bit). ( 140, -13, 139 ) /waypoint 140.03, -12.64, 138.75
In order to gain entrance to the Keep, you will need to kill crocodiles the moat until you get the brief cinematic, then kill Viletooth (crocodile) in the north end of the moat. Alternatively you can kill orcs around the outside of the moat, be sure to get the timed quest to kill 100 orcs. Then you can access the sewers.