Some have said this doesn't unlock....be sure to let credits roll all the way to the end to get the story mission complete screen. Trophy and the interceptor car unlocks at that screen.
Press x
*** Spoiler - click to reveal *** Your mission is to go after Scrotus' convoy. First eliminate all the escorts, then pull shields off the side of his ride, then poon his dumb ass. Afterward you'll have to fight him some more, but I'll let you experience that for yourself. *** Spoiler - click to reveal ***You will get the magnum opus back with chum bucket, a new black jacket (50% less damage from fist and weapon attacks) and a legacy double barrel shotgun with faster reload
So, now go kick some ass!