Speak to him to get flagged to use the Mountain Sewer portal beside him.
Show him the Ancient Arbalest for a one time bonus of 950,000xp.
Lore & Dialog
Warden tells you, "Near Tou-Tou, the home of one of Dereth's great lighthouses you'll find a sewer in the mountains."
Warden tells you, "This Mountain Sewer contains a great number of Zefirs, these are devious spell slinging creatures."
Warden tells you, "You can use the walls of the sewer to block the War Magic spells the Zefirs cast."
Warden tells you, "Delve deep into the sewer and locate an Ancient Arbalest and return to me with it, that is if you want my reward of course."