grim
inexorable
relentless
stern
unappeasable
unforgiving
unmitigable
unrelenting
adamant
adamantine
avenging
cast-iron
cruel
deadly
dour
firm
flinty
grudgeful
hard
hard-core
immitigable
immovable
immutable
impliable
impropitiable
inelastic
inextinguishable
inflexible
intractable
intransigent
iron
irreconcilable
merciless
mortal
pitiless
punitive
punitory
rancorous
retaliatory
revanchist
revengeful
rigid
rigorous
rock-ribbed
ruthless
steely
stiff
unaffected
unalterable
fierce
imperious
indomitable
malignant
remorseless
resolute
stubborn
sullen
unreasoning
vindictive
merciful