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")
brutal, unrelenting -- (harsh; "the brutal summer sun"; "a brutal winter")
persistent, relentless, unrelenting -- (never-ceasing; "the relentless beat of the drums")
(TTS Sound)
