The Halo Energy Sword by Mattel is already a wonderful cosplay prop. It has 11 LEDs, sound effects, and 2 modes. You can even put on a better show with more lights. Adafruit has shared a guide that shows you how to add more lights to the sword by replacing the LEDs with NeoPixel LED strips.
You are going to need an Adafruit 5V Trinket, Flex Perma-Proto, and 2 NeoPixel LED strips to get started. More info is available here.