Complex sentence: The king sent back that they should care for the child nonetheless, but the devil got at that letter too, and once again changed it, saying that they should kill the queen and the child and keep the queen's heart as proof.
Split into simpler sentences: The king sent back that they should care for the child nonetheless, but the devil got at that letter too and changed it once again. His letter said that they should kill the queen and the child and keep the queen's heart as proof.