Mu-onna

Mu-onna
 In Japanese folklore a Mu-onna ( 無女 lit."nothing woman"?) is a type of yōkai, a supernatural monster. A Mu-onna is a vengeful spirit of a mother who lost her child to famine or war. She protects children in danger, but may also try to merge or absorb them. They can look into the child's soul to find information while merging with them.

 To gather the information, or to merge with them, the Mu-onna must cast a spell to put the child's soul to sleep.

Since the Mu-onna is made from a mother's tender feelings, she may be willing to allow herself to be destroyed for the sake of the child