Murkblood Overseers can be found in Netherwing Ledge, Shadowmoon Valley deep in the mines, somewhere near the flayers. His hand is needed for  and he himself cuts it off and gives it to you to keep up your disguise.
Conversation with the Murkblood Overseer Edit
<The Murkblood overseer grunts.>
They sent you to kill me, eh? So predictable... Creatures ruled by terror are all the same.
But you... You are not one of them...
You are no more an overseer than I am the king of Stormwind. Yes... You are the one they speak of.
The Netherwing. They speak to us. They offered us peace and protection. Something the broken have never truly felt. We accepted their offer and assisted Toranaku in rousing the creatures of this mine - at great personal cost to us. Many of my brothers gave their lives for this offensive. We were attempting to make the mine uninhabitable, forcing the Dragonmaw to relocate and ultimately move off of this island.
The Dragonmaw are corrupt. They are strip-mining this island of all natural resources and using those resources to supply Illidan's armies out of the Black Temple. They take much for themselves, however, and sell whatever they have hidden away to the highest bidder.
We do not know. We believe that the Black dragonflight is involved as are some independent third parties. That is why you are here, <race>. You will unravel this mystery from the inside and bring redemption to Netherwing.
And now... I can only assume she asked you to bring back my hand.
I gladly make such a sacrifice if it means the downfall of the Dragonmaw.