Synonyms of the word

unforgiving

• adj. Unwilling or unable to forgive or show mercy.
• adj. Having no allowance for weakness.

grim

• adj. dismal and gloomy, cold and forbidding.
• adj. rigid and unrelenting.
• adj. ghastly or sinister.
• adj. (Britain, slang) disgusting; gross.

implacable

• adj. Not able to be placated or appeased.
• adj. Adamant; immovable.

inexorable

• adj. Unable to be persuaded; relentless; unrelenting.
• adj. Impossible to stop or prevent; inevitable.
• adj. Adamant; severe.

relentless

• adj. Unrelenting or unyielding in severity.
• adj. Unremitting, steady and persistent.

revengeful

• adj. Vengeful, vindictive.

stern

• adj. Having a hardness and severity of nature or manner.
• adj. Grim and forbidding in appearance.
• n. (nautical) The rear part or after end of a ship or vessel.
• n. (figuratively) The post of management or direction.
• n. The hinder part of anything.
• n. The tail of an animal; now used only of the tail of a dog.
• n. A bird, the black tern.

unappeasable

• adj. not able to be appeased or satisfied.

unrelenting

• adj. Not relenting; having no pity; not being or becoming lenient, mild, gentle, or merciful; unyielding; inflexibly…

vengeful

• adj. Vindictive or wanting vengeance.

vindictive

• adj. Having a tendency to seek revenge when wronged, vengeful.
• adj. (obsolete) punitive.