adjective
implacable
im-PLAK-uh-bul/ɪmˈplækəbl/
Unable to be calmed, softened, or stopped.
He remains an implacable enemy, unmoved by every offer of peace.

Etymology
The negative prefix reverses placabilis, “easily appeased.” Implacable therefore describes hostility or determination that efforts at soothing or conciliation cannot soften.
In context
of bodily sensation, as if she had been in very truth one of those disembodied spirits of Mrs. Johns' spirit world, driven through the solitude of the ages by the implacable decree of some incalculable malignant force called immortality.
More examples
- No apology can soften her implacable anger.
- The tenants face an implacable landlord who refuses to negotiate.
- She pursues the truth with implacable determination.
Forms
implacable
implacably adverb — Without being calmed, softened, or stopped.
implacability noun — The quality of being impossible to appease.
Similar words
- relentless
- Emphasizes continuing without easing rather than resisting attempts at appeasement.
- inexorable
- Emphasizes being impossible to prevent or change, rather than soothe.