Merge the content of the archive with your GTA: San Andreas installation folder (C:Program FilesRockstar GamesGTA San Andreas).
Replace "poolcue.dff", "poolcue.txd", "teargas.dff" and "teargas.txd" inside the file "gta3.img", located in the GTA San AndreasModels folder. You can use IMG Tool.
EX Rumia: many Touhou fans say that if you took the ribbon off Rumia's head, she'd become a demon; seems it was a true story.
Darkness: create an enormous black sphere which will swallow in darkness everyone around you.
Dark shot: shoot darkness blobs which will swallow the people they hit.
Mazinken: hit the ground with the sword and create a destructive darkness trail.
Moon lightray: a giant explosion will fall from the sky.
Known issues: because everything can't just be fine.
I took the liberty of editing some scripts, but please keep in mind that they are minor fixes or compatibility patches - You can download the original ones from the author's website, though.