Humans think that demons are trying to corrupt them and overthrow the gods, so why not do it. Go agnostics. The gods only grow more powerful as time passes by, while demons grow less slowly by comparison, so either spread a plague/ global cataclysm that will ravage the population, or kill the gods now and forever sunder their boundless faith. If a god is killed their faith will be shaken, and if all are killed any new god will forever hold a kernel of doubt within their followers. They will be shown to have limitations and demons will be able to take their place as the masters of man. The god’s will no longer protect the names of their followers and we can summon the souls of mortals in any form we desire. We can rule the mortals forever from the abyss as their existence on Astlan will turn into a living hell, and then into purgatory once more as people adapt.