Jyu-Oh-Sei

In 2436 A.D., young twins Thor and Rai are living on the space colony Juno. Their entire lives are turned upside down when one day their parents are murdered, and then they are kidnapped and ushered to the forsaken "planet of the beast king" by unknown masked men

(Source: TOKYOPOP)

Note: Includes the two-part story "Death Game".