The Gods of Mars - (version 3) - Edgar Rice Burroughs

After John Carter's arrival, a boat of Green Martians on the River Iss are ambushed by the previously unknown Plant Men. The lone survivor is his friend Tars Tarkas, the Jeddak of Thark, who has taken the pilgrimage to the Valley Dor to find Carter. Having saved their own lives, Carter and Tars Tarkas discover that the Therns, a white-skinned race of self-proclaimed gods, have for eons deceived the Barsoomians elsewhere by disseminating that the pilgrimage to the Valley Dor is a journey to paradise. Most arrivals are killed by the beasts of Valley, and the survivors enslaved by Therns. (Summary by Wikipedia)