Combine cake mix, pudding, eggs, sour cream, oil, and water in a mixing bowl. Mix well by hand.
Grate the dark chocolate bar. Then, gently fold in remaining ingredients and scrape sides of mixing bowl with spatula.
Grease and flour a 10-inch bundt cake pan. Spoon batter in and bake for 1 hour to 1 hour 15 minutes or until toothpick inserted near center comes out clean.
Allow the cake to cool on a wire rack for one hour before inverting onto a cake plate. Allow to cool completely before serving, about 2 more hours.
Recipe by Sweeterville at https://sweeterville.com/2014/05/twilight-delight-chocolate-bundt-cake/