Implacable.
ETYM Latin inexorabilis: cf. French inexorable. Related to In- not, and Exorable, Adore.
Not to be persuaded or altered; firm; unyielding; unchangeable.
Unalterable; relentless.
Unmoved by appeals for sympathy or forgiveness; insensible to the distresses of others; destitute of tenderness; unrelenting; unyielding; unpitying
Not relenting; unyielding; stern.