Aliens Vs Predator

This franchise originally started separately back in 1979 (Alien) and 1987 (Predator) as individual films concerning the sci-fiction genre. The Alien franchise focused  in the far future where space travel was possible, and in order for earth to continue its way of life colonization and planetary mining began. As far as
the future history is concerned, no contact with any alien lifeforms have ever been established. In 2122 however, the Xenomorph is found on LV-426 and begins to directly plague earth.

Yet the contact with the Xenomorphs is really not anything new to earth, in earth ancient past as far as 1.6million years ago xenomorphs did roam the earth. Furthermore there was evidence alien activity that the government had been observing and research, that of a species dubbed "Predator". The possible influence and application of alien technology came to the attention of several corporations (Weyland and Yutani).
The Weyland corporation funded and tracked activity concerning the predator species and discovered an ancient temple under Antarctica contain both Predator and Xenomorph artifacts. In the coming years, Weyland and Yutani united combine their efforts and research, becoming a mega-corporation.

The series soon became intertwined with each other. The relationship between the Predators and Xenomorphs has been established, as one has the Ultimate Hunter and the Ultimate Prey. Humans just so happened to be a species in which bring about an optimum Xenomorph. Even humans themselves provide prey for the Predator species, as humans are often resourceful and rather inventive.

Already some fans have created D20 variants for each of these creatures.  If you are interested in a Aliens D20 book click here, and Predator D20 series click here.