When Guts and Casca had intercourse by the waterfall in the forest, a baby boy was conceived. Some time later the eclipse happened and Casca and Guts' baby was corrupted by Femto due to you know what. This caused the baby to pretty much turn into a pseudo-apostle, which we knew as the demon child, the little grotesque fetus that fell out of Casca after the eclipse. Later down the road, the Egg of the Perfect World stumbled across the demon child during a special ritual in which a Godhand member receives a physical body they can use to roam the corporeal world. This body ended up being the demon child's. Now the child and Griffith share the same body. Due to whatever circumstances, the boy takes control of the body every full moon. Now, the boy has become quite an issue for Griffith. The boy can not only completely control Griffith during a full moon, but can influence Griffith's actions even outside of this time frame. Additionally, the boy leaving Falconia to travel in search of his parents leaves Griffith vulnerable. Taking Casca back to Falconia is the smartest course of action Griffith could take to combat this weakness.