Miasma

A village that feeds a frozen lake their dead to harvest the methane, suffers the loss of most of their people. The remaining 3 encounter the last survivor from another village that also feeds their dead to a lake. The stranger tells them that one season there were no dead to feed the lake, and their dead ancestors rose from the lake and killed everyone. As night falls, the rational villagers do not fear the undead, but they all ignorant of something just as horrible...