The Cranassian is the name given to an alien race and the main antagonists of the main Raiden series that overrun Earth, but are constantly having their units destroyed by the mechanical Earth guardians, the Raiden mk-II, and the Raiden mk-II ß.
According to The Raiden Project, the Cranssians' homeworld was Cranass, hence their name. The only well-known feature about the Cranassians are that they use Red Crystals to generate their heavy weapons. Because of the destruction of one of the Cranassian starships, Earth is now merging it's technology with that of the Cranassian, which means Earth now has it's own military units with Cranassian weaponry.
In Raiden V, it is revealed the actual Cranassians have already been extinct for a very long time while the Energen Crystals contain their souls and still carry their will. Whenever the Crystals arrive on a inhabited planet, they will infect the planet's weapons and slowly change the composition of the planet's atmosphere, until the planet becomes uninhabitable and all life on it is extinct. At the same time, they will also accumulate water and, once life is extinct, begin to restore the atmosphere so that life on the planet can begin anew.
The Cranassians have made their appearances in all the main Raiden titles. They would seemingly rely on the Red Crystals as their source of military units. The Crystals actually infect Earth's military weapons, thus taking control over them.
The following below is snapshots of the Raidens battling the Cranassian mechs.