I have a strange theory; Skull Knight tells guts that what he desires might not be what Casca desires when they speak of the elf island. I think this refers back to Guts drive and need for revenge against Griffith/Femto. Casca on the other hand seems like she wants her child back. I think the dilemma will present itself when we realize that to kill Griffith Gut’s also has to kill the moon child/ Gut’s and Caskas child. It makes senses the although the child is the offspring of Gut’s and Casca he is inevitably connected to Femto and the and astral world because of Caskas rape during the Eclipse. And During the second eclipse when the Behleit Apostle uses the demon child’s body to produce Griffith's new form. That’s my theory.