Prepare cake according to package directions. Make 12 cupcakes and 1 round cake. Save cupcakes for later and cool cake completely.
Mix together milk and pudding until smooth. Refrigerate for 5 minutes or until set.
Using a bread knife, cut the round cake in half to create two rounds. Place one round into the bottom of the trifle bowl. Top with half of the pudding and spread evenly. Add half of the whipped topping on top of the pudding. Coarsely crush 10 cookies and sprinkle on top of the whipped topping.
Repeat layers, ending with the whipped topping. Crush 15 cookies into fine crumbs for dirt.
Add water and food coloring to a bowl and mix well. You want a very concentrated color green. Add to coconut and mix well to evenly coat and create green coconut.
Place skeleton on top of whipped topping and shape as desired. Place crushed oreos around skeleton to create dirt. Sprinkle coconut grass around the gravesite.
Add RIP onto a Milano cookie with sparkle gel and press into the dirt to create a gravestone. Decorate with candy corn, candy pumpkins and candy rocks. Serve immediately or cover and refrigerate until ready to serve.