In a story I'm working on, humans unknowingly bring forth an apocalyptic scenario when they land on a planet and disrupt the balance between that planet's inhabitants. An ethereal race cleanses the dark side of the planet of a bacteria that produces methane gas, preventing temperatures from rising by greenhouse effect. If the temperatures rise sufficiently, the bacteria spreads. In return, the organic inhabitants offer their souls as sustenance to the ethereal just before natural death so life can flourish. Before this process, the ethereal were savage and stole organic life force from other species, almost wiping the planet of all life. The inhabitants of the planet lived harmonically in balance until the humans arrived. Humans eventually populate the world and, of course, start to take everything for their own. Unbeknownst to them, the ethereal don't mention the bacteria and, after thousands of years, the humans are forced to deal with the bacteria. But, its too late so they have to flee and the planet's natural inhabitants get their planet back.