I played Rygar on NIntendo for awhile but I never beat it. I know I would‘ve eventually.
The arcade game begins with the following introduction:
In the arcade version's story, the year 19XX sees the rise of dread creatures, not seen since before the Common Era, led by Ligar. Dominating the world, Ligar and his forces oppress humanity. The only thing that preserves humanity's will to survive is a prophecy that Ligar's rule will be opposed by the return of a long-dead warrior of Argos. That warrior does return to the world of the living, and he alone can save humanity.
Information gleaned from console manuals reveals that the evil being Ligar has taken over the land of Argool, and Rygar, a dead warrior who has risen from his grave, must use his Diskarmor to stop him. In console versions clues and limited dialogue are given in the form of large, sage-like men encountered in green stone temples throughout the game.
In the Japanese original, references to "Ligar" and "Rygar" are one and the same because the Roman syllables "Li" and "Ry" come from the same Japanese character, リ. In this version, the hero is only referred to as "The Legendary Warrior", while both "Rygar" and "Ligar" refer to the main villain.