Scoop flesh from one avocado half and add to the bowl of a food processor; pulse until blended. Add garlic and lemon juice; pulse until smooth. Season with cumin. Scoop out flesh of two other avocado halves, add to the food processor, and pulse until well combined and guacamole reaches desired consistency. Transfer guacamole to a bowl.
Stir in diced tomato and season with salt, cumin, and lemon juice.
Take the reserved avocado half and carve out a skull face, cutting all the way through the skin and flesh to expose the stone.
Serve the guacamole with the avocado skull on top and tortilla chips around the edge.
