Preheat oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.

Prepare the cake according to the package directions. Combine the cake mix with water, oil, and eggs. Mix well with a whisk until smooth.

Pour the cake batter into the prepared baking dish.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely.

In a medium bowl, combine the instant pudding mix and milk. Whisk until smooth. Refrigerate until the cake is cool.

Once the cake is cool, divide the pudding into two separate bowls. Add green food coloring to one bowl and orange food coloring to the other. Stir until evenly colored.

Using the end of a wooden spoon or a chopstick, poke holes all over the cooled cake, leaving a little space between each hole.

Carefully use a spoon or a clean cooking syringe to fill the holes with the green and orange puddings. Alternate colors for a marbled effect.

Place all Oreo cookies, including the cream filling, in a food processor. Pulse until you have fine Oreo crumbs.

Spread an even layer of whipped topping over the pudding-filled cake.

Generously sprinkle the Oreo crumbs over the whipped topping, ensuring the entire layer is covered to resemble dirt.

Break off the bottoms of the Milano cookies to create flat tombstone shapes.

Melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring in between, until completely melted.

Place the melted chocolate into a piping bag or a plastic bag with a tiny corner cut off. Write sayings and dates on the Milano cookies to create tombstones.

Decorate the cake to look like a spooky graveyard. Arrange the Milano cookie tombstones, gummy worms, candy pumpkins, Peeps ghosts and cats, and any other Halloween candies. Finish with Halloween sprinkles.

Chill the cake in the refrigerator for at least 2 hours before slicing and serving.