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Well, a few assumptions can be made. A plot twist: the restoration of a peaceful life and its subsequent collapse just as Guts finally finds his place. How did he survive after the festival? Perhaps Griffith himself let him live—a game of cat and mouse; he released him, the swordsman was tormented by nightmares, and later, the city awaited the appearance of the Black Swordsman. This is where everything went wrong according to Griffith's plan; he lost to Guts (by the way, he lost to him when Guts left the band in winter to seek the embodiment of "his dream," perhaps wanting a rematch). The army that was destroyed was enough for the rebirth of the hell-prince, and judging by the first (meaning—last) episode, Guts was avenging everyone; as for Casca, nothing was heard about her, and it would be correct to assume that she also perished, but one thing is confusing: the game of the same name. If according to the manga she remained with Griffith, then clearly not for long.