Alien remains a one-of-a-kind nightmare. There’s just no replicating the dread of Ridley Scott’s 1979 deep-space thriller. James Cameron knew that, and smartly did his own thing with Aliens, fashioning a sequel that traded the original’s ruthless minimalism for bug-hunt spectacle. Scott knew it, too, judging from the direction in which he took the franchise with a pair of prequels, Prometheusand Alien: Covenant, which were heavier on philosophy and mythology than terror. Some of the other entries in the series, including this week’s Alien: Romulus, have gestured toward the primal simplicity of the first, pitting a small group of desperate people against that slimy, sleek killing machine from beyond the stars. But all of them are echoes.