“...And you, Human, must follow our words, lest we summon up every curse that lies buried beneath these sands and pour them upon you and your children, and their children, and in turn their children, for so many generations that your blood will surely never survive.”
Nearly every primitive culture has had rituals and celebrations to guarantee the proper passage of the seasons and to ensure the fertility of crops and animals. Oversight of these ceremonies was generally the provenance of local kings or priests.
Suppose that the adventurers dispatch one of these fellows. The local peasants may become hysterical, fearing famine and death will stalk the land. Alternatively, they may want one of the new heroes to become king. For a while, this can be a good thing, but the first time that the crops fail, the superstitious locals will want to sacrifice their new leader.