Step 1: Prepare your brownie base by baking the brownie mix according to package directions in a 9x13 inch pan. Allow them to cool completely, then cut into 1-inch cubes.
Step 2: In a large bowl, whisk together the instant chocolate pudding mix with 4 cups of cold whole milk vigorously for about 2 minutes until it starts to thicken. Refrigerate for 5-10 minutes to set further.
Step 3: Crush your Oreos by removing the cream filling from 25-30 cookies. Place the dark chocolate cookies into a food processor and pulse until fine crumbs form, or crush in a zip-top bag with a rolling pin. Set aside.
Step 4: Begin assembling the trifle layers in a large clear trifle dish. Place half of the brownie cubes in an even layer at the bottom.
Step 5: Carefully spread half of the prepared chocolate pudding over the brownie layer, smoothing gently.
Step 6: Next, spread half of the thawed whipped topping over the pudding layer.
Step 7: Sprinkle about one-third of the crushed Oreo crumbs over the whipped topping.
Step 8: Repeat the layers: add the remaining brownie cubes, then the remaining chocolate pudding, and finally the remaining whipped topping.
Step 9: Sprinkle the remaining crushed Oreo crumbs generously over the top of the trifle to create a 'dirt' layer.
Step 10: Decorate your Chocolate Spider Trifle by arranging edible candy eyes on the 'dirt' layer and adding plastic spiders or creating chocolate 'spider webs' with melted chocolate.
Step 11: Cover the trifle loosely and refrigerate for at least 4 hours, or preferably overnight, to allow flavors to meld and layers to set before serving.