In a world of mythical creatures the "Grim Reapers" or Reapers are made empty carring the mark of death on their shoulder, until they meet their soulmate, that's when everything Axel knew changed.
A human makes a deal with a witch to save his dying father. The witch agrees in exchange for the human's first born child.
(Alternatively, the human may or may not recommend having feelings for witches who ask for other people's children.)