Slice one of the avocados in half; reserve the side with the stone in it for decoration. Scoop the flesh out of the half with no stone and place in a blender or food processor. Add garlic and lemon juice; pulse until smooth.
Add cumin and all the flesh from the second avocado; pulse until well combined and the guacamole reaches the consistency you prefer.
Stir diced tomato into the guacamole. Adjust to taste with salt, cumin and lemon juice.
Take the reserved avocado half and carve out a skull face, cutting all the way through to expose the stone.
Serve the guacamole in a bowl with the avocado skull on top and tortilla chips around the edge.