You get this quest by starting the Semper Invicta quest and by talking to Knight Rhys. This quest isn't as grandiose as it sounds; you don't have to rid the entire Commonwealth of hostiles. You simply need to walk a couple blocks north from the police station to the subway under College Square.
Eliminate all the ghouls in first couple rooms of the subway to complete the mission. The main target is the Feral Ghoul Stalker, indicated by the waypoint.
Before you leave to report your success to Knight Rhys, you can take your time in scavenging for items. One valuable collectible you can find is the Live & Love magazine, which gives companions a 5 percent boost in inflicting damage. Check out our Magazine Collectibles guide for details.