Place ½ cup of the simple syrup, ice, gin, pomegranate juice, lime juice, and blackberry liqueur in a cocktail shaker. Close the shaker and shake well to combine.
Divide the raspberries and blackberries between 4 cocktail glasses filled with crushed ice. Strain the cocktail mixture over the fruit. Top it off with your favorite Prosecco. Garnish with a spooky spider and serve!