This is a text about ancient Babylon. Nothing in context gives me real clue of "sinister" meaning here, used as a noun — except that it gives an analysis of necromancy in then ancient city. Should I understand "sinister" here as "omen" ?
Such stories reflected not only the faint aura of the sinister that had always clung to Babylonian learning, but also an ambition on the part of Jewish scholars to lay claim to its inheritance.
Quelles sont les possibilités pour interpréter "sinister" ici ? …"une vague atmosphère de présage" ? Lié au fait que les Babyloniens pratiquaient la nécromancie. Car "sinistre" seul ici ne veut rien dire.
Regarding the meaning of the English sentence, see the related question on English Language Learners. Note that sinister in the English sentence should be understood as an adjective: “the faint aura of the sinister” means “the faint aura of something that is sinister”.