Unrelenting

cruel grim hard-headed implacable mortal pitiless relentless remorseless ruthless stern tough unsparing ceaseless constant continual continuous endless incessant inexorable perpetual persistent steady unabated unbroken unflinching unwavering unyielding bound dead set on rigid set stiff tenacious unbending brick-wall hanging tough iron-fisted intransigent stiff-necked bound and determined