This project has moved to Planet Minecraft.
This data pack adds a new boss monster; the Phantom Menace. The Menace is a larger than usual phantom, with a few tricks up its sleeve. The Menace will appear after a semi-random number of nights, if there are any other phantoms around, and will prevent you from sleeping when it is close to appearing.
You can fight it for rewards, or run from it. Escaping from it will allow you to sleep at night again, and delay its reappearance until another semi-random number of nights have passed. Killing it will give you some loot and experience points, as well as preventing phantom spawning except on full moons.
After you've killed the Menace, you can get phantoms to spawn every night again by renaming a firework "Menace" and using it. This will also cause the Menace to reappear eventually.
If you kill the Ender Dragon (after installing this data pack), every Menace you kill after that will drop an Elytra.
You can change how often the Menace appears by altering the maximum and minimum safe nights between appearances in the config file ("data\phantom_menace\functions\config.mcfunction") in the data pack zip file.
More information about data packs and how to use them can be found on the official Minecraft Wiki.