Demons and spirits served important functions in pre-industrial Slavic societies and were considered to be very interactive in the lives and domains of humans. Some spirits were benevolent and could be helpful in human tasks, others were harmful and often destructive. Examples of such spirits are Domovoi, Rusalka, Vila, Kikimora, Poludnitsa, and Vodyanoy. These spirits were also considered to be derived from ancestors or certain deceased humans. Such spirits could appear at will in various forms including that of different animals or human form. Some of these spirits could also participate in malevolent activity to harm humans, such as drowning humans, obstructing the harvest, or sucking the blood of livestock and sometimes humans. Hence, the Slavs were obliged to appease these spirits to prevent the spirits from their potential for erratic and destructive behaviour.