The Holy Therns were a vicious Martian cult populated by the therns. Inhabitants of the Valley Dor, they worshipped the "living goddess" Issus (who is actually merely an ancient Black Martian woman), and sought to keep her happy with human sacrifices. For centuries, they enthusiastically promoted the idea that the Valley Dor is the gateway to Martian Heaven and decreed that any Martian who had reached 1,000 years of age or merely tired of living should travel to the valley to either spend the rest of eternity in paradise or be reborn. When the unsuspecting pilgrims reached the valley, they found a beautiful land where fruit and water were plentiful... which also happened to be heavily inhabited by white apes and plant men. Those who survived those creatures ended up enslaved by the Holy Therns, who would work them to death before feeding them to Issus or the Tenth Cycle. The Holy Therns' downfall came in 1886, when John Carter suddenly returned to Mars.