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) 
Includes a two-part short called "Death Game".

Thor and his twin brother, Rai, are cast off to a forsaken planet by unknown masked men after witnessing the suspicious death of their parents. Given only a beam knife and a smelly satchel, they wander together in search of civilization on the primarily plant based world. They soon discover the plants have the advantage. Only the strong survive; the ones who throw away their humanity and become the beasts humans once were.
 
(Source: Anime News Network)

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