This quest is given by Lavic N'Gam in South Freeport after you
complete Like a Ring in a Haystack.
You are sent to harvest armadillo dens in the Commonlands until you find "the scholar's tool". Next, you must kill Dervish Go-Betweens until you find a ring. Finally, you need to kill Scourge Rats for some bile and Fallen Guardians for their rusty weapons.
Once you have all the items, you must combine them in "a barrel near the house of a Dervish Cutthroat". You can find this by the house across from the Captain's Deck ( 1341, -40, 260 )
Return to Lavic N'Gam to receive Lavic's Backup. This will also open up the No More Witnesses quest.