Melt the chocolate chips, peanut butter and butter together in a saucepan over low heat. Pour over the cereal and toss to coat thoroughly. Add 4 cups of the chocolate wafer cookies and toss to coat. Pour into a 9-by-13-inch casserole dish, then sprinkle the remaining 1 cup cookie crumbs over the top.
Add the melted chocolate to a piping bag fitted with a fine round tip.
To make gravestones, pull apart the sandwich cookies and remove the frosting. On the flat side of the cookies, add a border of chocolate around the edge and "RIP" at the top of each. Decorate the graveyard with marshmallow ghosts, gummy worms and gravestones.