I don't think the beast of darkness will consume Guts' body, but rather his mind. Guts has a dream when on the journey to Elfheim where the beast of darkness prophesizes that all of Guts' companions will die horribly and at that moment the beast will take over his mind and he will scream in ecstasy and murderous rage to sink his 'fangs' into the white hawk (Griffith). If this is foreshadowing by Miura then I feel a certain sense of dread for the future of Berserk, a repeat of the eclipse in some way.