"I need more bodies! You think my summoners can just conjure up a demon whenever it pleases them? We must have blood — fresh blood, mind you — and human flesh to bolster our magic. I will be expecting a reply from you shortly."— The Grand Inquisitor(src)
The Grand Inquisitor, Master of Pain, was a high-ranking member of the Coven. He demanded additional bodies for his Dark Summoners' demonic rituals and threatened his subordinates when there were no villagers left to be found. It became a moot point however, as he was cut down by the Nephalem.
"Bring [him/her] to me so I may rend the flesh from [his/her] bones!"— The Grand Inquisitor(src)
He does not have to be killed to advance further in game, but most players choose to do so.