Complex sentence: Their mere existence made it impossible for Quan Chi to enter the temple where the amulet rested, but if he could recruit a human who stood a chance against the elements, then he could achieve his goal.
Split into simpler sentences: Their mere existence made it impossible for Quan Chi to enter the temple where the amulet rested. But, if he could get a human to enter, the right human, one who could perhaps defeat the elements, then he could achieve his goal.