implacable (adj.)

• adamant
• adamantine
• cast-iron
• cruel
• dour
• firm
• flinty
• grim
• hard
• hard-core
• immovable
• immutable
• inelastic
• inexorable
• inflexible
• intractable
• intransigent
• iron
• irreconcilable
• merciless
• mortal
• pitiless
• punitive
• punitory
• rancorous
• relentless
• retaliatory
• revengeful
• rigid
• rigorous
• rock-ribbed
• ruthless
• steely
• stern
• stiff
• unaffected
• unalterable
• unappeasable
• unbending
• unchangeable
• uncompassionate
• uncompromising
• unflinching
• unforgiving
• unmoved
• unrelenting
• unsympathetic
• unyielding
• vengeful
• vindicatory
• vindictive