Take a plastic glove and fill it with water. You can also fill one with a red beverage as well. Freeze it. After that take the glove off and you can have a floating hand in your punch bowl. The red one will look bloody.
You can also pour grenadine to the top of your punch bowl, which will look like blood when it drips down from the top of the bowl. Other creative garnishes: black licorice wheels as garnish or straws, candy lime slices, or gummy worms.