Step 1: Bake brownies according to package directions in a 9x13 inch pan. Let cool completely, then cut into 1-inch cubes.
Step 2: In a large bowl, whisk together instant chocolate pudding mix with 4 cups cold whole milk for 2 minutes. Refrigerate for 5-10 minutes to thicken.
Step 3: Remove cream filling from Oreo cookies. Place dark chocolate cookie wafers into a food processor and pulse until fine crumbs form. Set aside.
Step 4: Arrange half of the brownie cubes in an even layer at the bottom of a large trifle dish.
Step 5: Carefully spread half of the prepared chocolate pudding over the brownie layer, smoothing gently.
Step 6: 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: Generously sprinkle the remaining crushed Oreo crumbs over the top of the trifle to create a 'dirt' layer.
Step 10: Decorate with edible candy eyes and plastic spiders, or create chocolate 'spider webs' using melted chocolate.
Step 11: Cover the trifle loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow flavors to meld and layers to set.