Australian Aboriginal mythology Australian Aboriginal religion and mythology is the sacred spirituality represented in the stories performed by Aboriginal Australians within each of the language groups across Australia in their ceremonies. Aboriginal spirituality include ...

(specifically:

Yolngu The Yolngu or Yolŋu ( or ) are an aggregation of Aboriginal Australian people inhabiting north-eastern Arnhem Land in the Northern Territory of Australia. ''Yolngu'' means "person" in the Yolŋu languages. The terms Murngin, Wulamba, Yalnuma ...

), Mokoi (lit. "evil spirit") is an evil spirit who killed sorcerers who used

black magic Black magic (Middle English: ''nigromancy''), sometimes dark magic, traditionally refers to the use of Magic (paranormal), magic or supernatural powers for evil and selfish purposes. The links and interaction between black magic and religi ...

. Also known to kidnap children at night to eat them. The Murngin believed that death was rarely caused by old age and instead it was the work of a mokoi, who would bring about some sort of disease or fatal accident. The legend say he gave his life for the devil. He's in his debt and he's helping him. His main task is to kill magicians who take pleasure in black magic. He does this because the magicians take a pact and it leads to death. He eats the children to extract their elixir of life. He needs the elixir to strengthen himself, which is paradoxical because he's dead. Another thesis is, that he kills the children to lure the parents into the woods to inflict them harm.

Sources

https://www.godchecker.com/australian-aboriginal-mythology/MOKOI/ Australian Aboriginal legendary creatures {{Australia-myth-stub