Adjectives

(a)unforgivingunwilling or unable to forgive or show mercy“a surly unforgiving old woman”
(s)grim, inexorable, relentless, stern, unappeasable, unforgiving, unrelentingnot 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”