Actually Karni Mata was charan, she was an avatar of Maa hinglaj, Maa hinglaj is kuldevi of charan where Karni Maa had taken birth.
Since I am a son Maa Karni I think I am a eligible to answer it.
It was long story where Karni Mata sister son lakhan died in river kola Yat, all the family members request Maa to revive their son, Maa has taken son into cave usually where Maa use to do sadhana of awad mata, and call yamraj the God of death, and told him to give back life of his son, at that time yamraj told to Maa Karni that he is doing this, but what will happen if the future it happen again then Maa Karni said any person of my samaj die he will become a mouse, and as he complete his mouse life he again rebirth as a human being
Jai Maa Karni, jai awad Maa, jai Maa hinglaj