Achlys (ἀχλύς) refers to a mist, dimness, or murky film — the haze that falls over the eyes. In classical Greek it described the dimness of death descending over dying eyes. In the NT it appears once in Acts 13:11, where Paul pronounces blindness on the sorcerer Elymas: "Immediately mist and darkness came over him."
The achlys that fell on Elymas was divine judgment — a physical manifestation of spiritual blindness. This sorcerer had been "making crooked the straight ways of the Lord" (Acts 13:10). One who trafficked in darkness was visited with literal darkness. Yet Paul's own conversion included temporary blindness (Acts 9:8–9) before sight was restored — suggesting even this judgment could be remedial. Jesus came as the "light of the world" (John 8:12) to remove the spiritual achlys sin has cast over human hearts. Judgment enacted what sin does to every unrepentant soul.