Adjectives

(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”
(s)brutal, unrelentingpunishingly harsh“the brutal summer sun”, “a brutal winter”
(s)persistent, relentless, unrelentingnever-ceasing“the relentless beat of the drums”