Pretty much an enforcer in some ways. The book defines it as this:
"As the nights grow more and more violent and the cities fill with unknown Kindred, some princes have resurrected this ancient position. Essentially, the scourge patrols the borders of a princedom, seeking out and often destroying newcomers who have failed to present themselves. Caitiff, as well as the fledglings of the 13th, 14th, and 15th generations, have much to fear from the scourge. In some cases, even vampires who have followed protocol fall victim to the scourge, as princes reflexively react to fears of overpopulation and espionage. A few scourges are Assamite assassins under contract to a prince."