To account for the angel's limited powers and the potential absence of certain ingredients, the resulting holy fire glasses can indeed work not quite as intended. How this manifests is up to you. You can choose to have the effect fail intermittently at inconvenient times, the enchantment may allow the user to see the Hellhounds but cause everything else to appear blurry, or anything along those lines.
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To account for the angel's limited powers and the potential absence of certain ingredients, the resulting holy fire glasses can indeed work not quite as intended. How this manifests is up to you. You can choose to have the effect fail intermittently at inconvenient times, the enchantment may allow the user to see the Hellhounds but cause everything else to appear blurry, or anything along those lines.