Boxed cake mix, any flavor
Blackberries and Raspberries
Red decorating icing
Bundt Cake Pan
Three dishes to make gelatin in
Cutting circles in three sizes
You can start to see your bloody eyeball, right?
You can wait to fill the cake until right before serving, or you can keep it in the refrigerator so the Cool Whip doesn't melt.
Another option is to make a round meringue disk for the center. Just trickier to do.
Color a second dish of gelatin something close to black for the pupil.
Leave the third dish uncolored. (This step is not essential, but it provides some definition inside the pupil.)
Pretty scary, isn't it?