Before leaving the tower leading up from the sewers with Helchgam you will run into Sálvakh. If all the adds in the two rooms beside his are defeated, he will be no problem to take down. He's sitting at a table to the right just before the exit from the tower.
Quotes
"Intruders! Where did they come from?"
"I am proud to wear the robes of the Angmarim!"
"What a marvelous place Mordirith has given us."
"I have heard that Mordirith speaks with the Great Eye himself."