Special equipment: a 9 1/2-by-3 1/2-inch bundt pan, a pastry bag with a medium round tip, an offset spatula and two 8-inch bamboo skewers, cut in half
For the cakes: Preheat the oven to 350 degrees F. Spray a 9 1/2-by-3 1/2-inch bundt pan with nonstick cooking spray and dust with flour.
In a large bowl, combine 2 of the cake mixes and mix according to the package instructions. Pour into the prepared bundt pan and bake until a toothpick inserted into the cake comes out clean, 55 minutes to 1 hour and 5 minutes. Let cool in the pan until warm, then remove and transfer to a cooling rack. Spray and flour the pan again and repeat the process with the remaining cake mix. The smaller cake should bake for 40 to 45 minutes.
For the decorations: Mix the pudding according to the package instructions, adding the neon green food coloring. Hold in the refrigerator.
Slice about a 1 1/2-inch-thick slice off the flat end of the large bundt cake and set aside; this will become the top lip of the cauldron. Place the large bundt cake cut-side up onto a large platter or cutting board.
Slice a little off the flat end of the smaller bundt cake to make a level surface. Reserve the scrap pieces. Place the small bundt cake cut-side down on top of the large bundt cake. Slice a little off the rounded top of the smaller bundt to make a level surface and reserve the scraps. Fill the center hole of the stacked bundt cakes with the reserved scrap cake pieces.
Fill a piping bag fitted with a medium round tip with some of the chocolate frosting. Pipe some of the frosting onto the top of the stacked bundt cakes. Carefully place the 1 1/2-inch-thick piece of cake cut-side up onto the frosting to form the lip of the cauldron.
Starting about an inch from the edge, scoop out about a 1/2-inch-deep well from the top layer. This is where the pudding will go.
Pipe frosting around the entire outside of the cake and on the lip. Smooth out the frosting with an offset spatula. Carefully press the wafer cookie crumbs into the frosting. Brush off the excess crumbs that fall onto the platter.
Cut about half of the candy bars into varying lengths. Arrange the whole candy bars and pieces around the bottom of the cauldron cake to resemble logs. Arrange the red hard candy in between the logs to resemble hot coals.
Insert 2 skewer pieces in the cut sides of 2 of the donut halves. Press the skewers and donut halves into the side of the cauldron, just below the lip, to form handles.
Carefully spread the pudding inside the well on top of the cake. Arrange the gumballs and nonpareils in the pudding to resemble bubbles. Press the gummy eyeballs into the surface of the pudding so they are partially covered.