Cut the bottom portion (stem) of cauliflower flat to get a flat base so it will sit alone, being careful not to cut too deep as to make the head fall apart. Carve the top portion out to make a "bowl" inside the cauliflower head. (Keep the florets to serve alongside)
Prepare the dip by mashing the avocados and sprinkling lime juice on them, mixing together. Chop the onion, tomato, and jalapeno pretty small and fold into the mix along with the minced garlic. Add in the salt, cumin, cayenne pepper and fresh cilantro. Mix all of this together well and let it sit for about 30 minutes or more.
Fill the "cavity" of the cauliflower with the dip.
Drizzle red food coloring on the cauliflower to create "blood" I wrapped mine with some plastic wrap, which makes it look even creepier when the "blood" sticks to it.