implacable

adjectives

incapable of being placated
"an implacable enemy"
implacable
antonym
placable
is similar to
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"
grim, inexorable, relentless, stern, unappeasable, unforgiving, unrelenting
is similar to
incapable of being mitigated
"stern and unmitigable accusations"
unmitigable
see also
having or showing no mercy
"the merciless enemy"; "a merciless critic"; "gave him a merciless beating"
merciless, unmerciful