this stillness of life did not in the least resemble a peace. It was the stillness of an implacable force brooding over an inscrutable intention. It looked at you with a vengeful aspect. I got used to it afterwards;
Joseph Conrad. The Heart of Darkness
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"That is a failing indeed!" cried Elizabeth. "Implacable resentment is a shade in a character. But you have chosen your fault well. I really cannot laugh at it. You are safe from me."
Jane Austen. Pride and Prejudice (1813)
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Looks and faces were even more violent and furious than their words. They expressed the most implacable hatred.
Leo Tolstoy. Anna Karenina
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At dawn the mortar round started falling again, with the same monotonous and implacable regularity.
Melania G. Mazzucco. Limbo (Virginia Jewiss translation), p.276 (2014)