@DepthSailor
The child was made by Guts and Caska and prematurely aborted by Grifith and the child had to accept evil inside him (Grifith's cum) in order to "survive".
Survival not in the literal sense since he died or was never really alive to begin with, who knows, but he did manage to develop a sense of self even though he was never born and got some power from a demon god so he can do stuff like this.
@Wantagom thanks, now it makes sense, maybe the fetus didn't die, but since it's still weak it couldn't resist to protect itself from the evil, like when the ghosts try to possess Gutts, but since it's Griffith's power, it's from a different level
or that trash Griffith knew and wanted to make it his