Preheat your oven to 375°F (190°C). Prepare a 9-inch pie dish.
Roll out one pie crust and fit it into the dish, trimming excess dough from the edges.
In a large bowl, combine blueberries, blackberries, lemon juice, sugar, cinnamon, and cornstarch. Toss until the berries are evenly coated.
Pour the berry mixture into the prepared crust, spreading it evenly.
Roll out the second crust on parchment paper. Using a sharp knife, cut out Jack Skellington’s face—two large eyes, two small nose holes, and a wide stitched grin.
Carefully place the decorated crust over the filling. Press edges to seal.
Brush with beaten egg and sprinkle with coarse sugar.
Bake 40–45 minutes, until golden brown and filling bubbles.
Let cool at least 30 minutes before slicing so the filling sets.