Introduced early on in volume 1 and the heart of the drama in the last episode of volume 41... It's Guts and Casca's son, of course. In his early, misshapen form, we call him the "Demon Child". That comes from the title of episode 92 (at the beginning of volume 14), when we see his tragic birth.
In Japanese, it's 幼魔, pronounced ようま (YOUMA). 幼 means a young child or an infant. As for our old friend 魔, you must be familiar with it by now! In Berserk, it's typically used by itself to refer to the evil power wielded by the God Hand and imparted to the apostles. In this case however, it's simpler to translate it as "demon" or "demonic", to convey his supernatural nature.