austere, stern -- (of a stern or strict bearing or demeanor; forbidding in aspect; "an austere expression"; "a stern face")
grim, inexorable, relentless, stern, unappeasable, unforgiving, unrelenting -- (not to be placated or appeased or moved by entreaty; "grim determination"; "grim necessity"; "Russia's final hour, it seemed, approached with inexorable certainty"; "relentless persecution"; "the stern demands of parenthood")
stern, strict, exacting -- (severe and unremitting in making demands; "an exacting instructor"; "a stern disciplinarian"; "strict standards")