Preheat oven to 350°F. Prepare three 8-inch cake pans with parchment paper circles and grease the sides.

Mix dry ingredients: flour, baking powder, baking soda, and salt in a medium bowl. Set aside.

Cream butter, oil, sugar, and vanilla until fluffy, about 3-4 minutes. Add eggs one at a time, mixing well after each addition.

Alternate adding dry ingredients and buttermilk to the batter, beginning and ending with dry ingredients. Divide batter evenly into two bowls.

Color one bowl with red gel icing and fold in chopped chocolate and black gel color. Fold black gel into the other bowl.

Drop alternating spoonfuls of each batter into pans and swirl gently to create marbled effect.

Bake for 20-25 minutes until a toothpick comes out clean. Cool completely.

Prepare frosting: Beat butter, add powdered sugar, cocoa powder, vanilla, and water to achieve desired consistency. Mix in black gel for a dark frosting.

Assemble cake: Level layers, frost between each, and use a piping bag for blood drip effect with melted red candy melts mixed with heavy cream.

Decorate with sprinkles such as bones, bloody eyeballs, and blood drips. Chill until ready to serve.