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According to what I have researched, Wyverns were D Ranked Monsters, which were already terrifying enough to destroy entire villages and even threaten big cities if their numbers were more than three. However, it was stated that the Wyvern living in the Mountain far away wasn't a normal one, it had been mutated through the Demonic Miasma and had turned even stronger than normal Wyverns. Some catalogued it as a C Rank Monster, while some said it could even compare to a B Rank Monster depending in the intensity of the Demonic Energy.