Doctor costume
Fake blood
Surgeon's tools
Table
Sheet
Strobe lights
Victim mask/makeup
Body parts
Normal lights and switches
Optional ground beef

There is a bloody doctor standing over a victim lit by a single bulb. As the guests enter the room, the surgeon looks up and welcomes his new victims. Just then the lights go out, actors dressed as victims move in behind the group from a concealed location, and the strobe lights come on. The rest is history.