Darius Alpha is a promotional version of Darius Plus released in 1990 for the PC Engine. It was a mail-order exclusive, obtainable by sending in a pair of coupons found in the manuals of Super Darius and Darius Plus. There were only 800 copies of the card produced, making it one of the rarest, and the absolute most expensive, PC Engine HuCard ever made. This version is only a Boss Rush, where the player faces against all Plus bosses one after another.
Like Plus, Alpha features SuperGrafx support, which greatly reduces sprite flickering.
The boss rush
Darius Alpha plays much like the boss rush mode found in Super Darius, but with the boss count and order from Plus instead. Pressing Select on the title screen starts the game with a 4-minute time limit.