The phrase "nightmarish visions" refers to scary and unpleasant images or scenes that appear in someone's mind while they are asleep, like the bad and terrifying parts of a nightmare. Full definition
In his breakout role, Jake Gyllenhaal uses his haunted eyes to their greatest effect as the titular troubled kid, living in an anonymous late»80s suburb, where he's plagued by nightmarish visions of a man in a rabbit costume, ominously counting down to the apocalypse. (avclub.com)
It Follows / U.S.A. (Director and screenwriter: David Robert Mitchell)-- After a strange sexual encounter, a teenager finds herself haunted by nightmarish visions and the inescapable sense that something is after her. (collider.com)
That realistic air contrasts with the flashes of crimson, which feel all the more graphic, exploding in nightmarish visions of blood - soaked waterfalls and bat - covered cliffs. (vodzilla.co)