Darius Alpha is a bare bone boss rush, and the goal of the game is to face all sixteen bosses from Darius Plus in a row (can be played either in Normal or in a 4 minutes Time Attack mode).
This Darius game was a promotional release. Owners of both "Darius Plus" (HuCard) and "Super Darius" (CD-Rom) could mail a proof of purchase to Taito, and be entered into a drawing for the chance to receive one of the eight hundred copies of Darius Alpha (two hundred extra copies are believed to have been given away by Japanese magazines at the time). A letter enclosed with the game indicates that NEC received a total of 9762 entries!
Darius Alpha is one of the rarest game available for the PC Engine system.