Implacable

a.	1.	Not placable; not to be appeased; incapable of being pacified; inexorable; as, an implacable prince. I see thou art implacable. - Milton. An object of implacable enmity. - Macaulay.
	2.	Incapable of being relieved or assuaged; inextinguishable. O! how I burn with implacable fire. - Spenser. Which wrought them pain Implacable, and many a dolorous groan. - Milton.

Adj.

1.

implacable - impossible to placate; "an implacable enemy" Antonyms: placable - easily calmed or pacified

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