"The mist that was upon (the eyes) before." This is Poe's reference to Homer, Iliad 5.127, where Athena encourages Diomedes to join the battle by telling him
ἀχλὺν δ' αὖ τοι ἀπ' ὀφθαλμῶν ἕλον ἣ πρὶν ἐπῆεν,
ὄφρ' εὖ γιγνώσκῃς ἠμὲν θεὸν ἠδὲ καὶ ἄνδρα.
I've taken from your eyes the mist that was upon them before,
So that you may discern both god and man.
Unfortunately, Poe gets the gender of ἀχλύς wrong, misremembering it as masculine (ὃς) instead of feminine (ἣ). We have to understand that everyone in Poe's social class, regardless of linguistic ability, was forced to study Greek in school, and to memorize Homer among other classics. Poe at least took something from it, and was proud enough of his Greek to flaunt it in his work.