Darth Vader
Darth Vader's menace derives substantially from his seeming inevitability within the Star Wars narrative. When his TIE fighter appears behind the Rebel fleet, when his respirator echoes through Death Star corridors, escape appears impossible. The Sith Lord pursued Luke Skywalker across the galaxy with single-minded determination, cornered Princess Leia aboard the Tantive IV, and tracked the Millennium Falcon to Bespin with the patience of a hunting spider. His mechanical enhancements render him nearly impervious to conventional attack; his Force sensitivity alerts him to danger before it manifests. However—and this proves crucial—Vader can be defeated. Luke bested him in combat. The Emperor's lightning nearly destroyed him. His own son's compassion ultimately unmade his dark persona entirely.
Time
Time's inevitability operates on an entirely different register. It cannot be fought, negotiated with, bribed, or Force-choked into submission. No entity in the known universe has successfully evaded time's passage. Stars burn through their hydrogen and collapse. Civilisations rise, flourish, and crumble to dust. Mountains erode grain by grain into the sea. The very protons that comprise matter may themselves decay given sufficient aeons. Time does not pursue its prey—it simply is, and everything else occurs within its embrace. Where Vader must expend tremendous energy to maintain his menacing presence, time expends nothing whatsoever. It merely continues, indifferent to the desperate struggles of those caught in its current.