Heat oven to 350°. Grease bottom only of rectangular pan, 13 x 9 x 2 inches, with shortening.
Beat cake mix, cracker crumbs, water, oil and eggs in large bowl with electric mixer on low speed 30 seconds; beat on medium speed 2 minutes. Pour into pan.
Reserve 1/4 cup hot fudge topping. Drop remaining hot fudge topping by generous tablespoonfuls randomly in 12 to 14 mounds onto batter in pan.
Bake 40 to 45 minutes or until toothpick inserted in center comes out clean. Run knife around side of pan to loosen cake. Cool 15 minutes.
Spoon teaspoonfuls of marshmallow creme onto warm cake; carefully spread with knife dipped in hot water. Drop small dollops of reserved hot fudge topping randomly over marshmallow creme.
Swirl topping through marshmallow creme with knife for marbled design. Cool 2 hours. Store uncovered at room temperature. (Once cut, store cake in the pan, uncovered, with plastic wrap pressed against cut sides.).