Make sure you download both the resourcepack and the datapack.
This datapack/resourcepack adds a new, more sinister form to dex #666, Vivillon. The form has a couple extra moves, including two new custom moves, Sinful Strike and Brimstone. Sinful Strike is a dark-type special move that has a chance to poison the target. Brimstone is basically a dark-type version of hyper beam.
To obtain this new form, use a Dusk Stone on a Spewpa.
Also comes with shiny support :D