[adj] impossible to placate; "an implacable enemy"
grim, inexorable, relentless, stern, unappeasable, unforgiving, unmitigable, unrelenting
placable
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, puniti