Is it an adverb, pronoun, or something else?
If it's a declension of весь, which case and gender is it in here?
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In this sentence всё is a pronoun. While English speakers use all in place of an adjective and everything in place of a noun, in Russian всё may stand for both (BTW. English All, as a noun only, corresponds to Все in Russian).
As Russian pronouns do decline, всё is a neutral gender singular of весь in both Nominative and Accusative.