Carry a fresh (unopened/new) box of Kellogg’s Corn Flakes with you, to the costume party.
When someone comes up to you and asks, “What the Hell kind of costume is THAT?” you reach into your pocket, take out some kind of KNIFE, and in front of those assembled, begin stabbing the box that you are carrying.
Then announce to the crowd, LOOK! I AM A CEREAL KILLER!”