Imperium, known in Japan as Kidou Soukou Dion ("Mobile Armor Dion"), is a vertical shoot 'em up from Jorudan and Vic Tokai. The player controls a flying mecha suit known as Imperium (US) or Dion (JP), depending on the region. While the player only has one life (with limited continues), they have a health bar that allows for up to five hits. Instead of acquiring power-ups in the field, Imperium/Dion earns XP for destroying enemies and can eventually level up and earn stronger weapons and abilities through that system.
The game features an opening text crawl that explains that humanity has all but abandoned a region of space which is now occupied by a martial race of high-tech robotic lifeforms and a slave race of mechanical creatures that serve them. The slave race intends to rebel, but can only do so with a discovered suit of powered armor that only a human being can ride. However, the game is set during 2027 and the additional material suggests that an alien invasion of Earth has occurred and humanity intends to repel the invaders with a last ditch effort involving the mech suit Imperium/Dion.
This Imperium is not to be confused with the 1990 Electronic Arts strategy/adventure/simulation game Imperium, despite the similar sci-fi setting.