Make me a cocktail that I can use in drinkable blood bags for Halloween. I have rum, gin, vodka, elderflower liquor, tequila, blue curaçao, grenadine, and mixers.

A drinkable blood bag is a fantastic idea for a Halloween celebration, and you're well-equipped with a variety of spirits and mixers to create something exciting. From your list of ingredients, I recommend creating a cocktail inspired by the mysterious and vibrant Devil’s Blood. This cocktail has a thrilling presentation and complex flavor profile, perfect for a 'blood bag' theme.

Start by using vodka as your base spirit, which will give the drink a smooth undertone. Add a dash of grenadine for that unmistakable blood-red color and sweetness. Although you don't have Jagermeister and spiced rum like in the original recipe, you can replace these with a dash of elderflower liqueur to bring a fragrant complexity and some of your available rum to maintain the balance of warmth. For fruitiness and balance, throw in some of the mixers you have, like cranberry juice, which is often a great pairing for fall-themed drinks, or any other juice you have on hand. Mix everything together, and you’ll have the perfect scary and flavorful cocktail to sip from a blood bag. Imagine the delight and chill it will bring to your Halloween festivities!