All 3 Uses of

Reign of Terror
in
The Count of Monte Cristo

"Ah," said the Marquise de Saint-Meran, a woman with a stern, forbidding eye, though still noble and distinguished in appearance, despite her fifty years—"ah, these revolutionists, who have driven us from those very possessions they afterwards purchased for a mere trifle during the Reign of Terror, would be compelled to own, were they here, that all true devotion was on our side, since we were content to follow the fortunes of a falling monarch, while they, on the contrary, made their fortune by worshipping the rising sun;"
Chpt 5-6

Reign of Terror = the ten-month period during the French Revolution when thousands were formally executed (1793-94)

"'Tis true, madame," answered he, "that my father was a Girondin, but he was not among the number of those who voted for the king's death; he was an equal sufferer with yourself during the Reign of Terror, and had well-nigh lost his head on the same scaffold on which your father perished."
Chpt 5-6

Definitions:

(1) (Reign of Terror)
the ten-month period during the French Revolution when thousands were formally executed (1793-94)
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

When not capitalized, reign of terror more likely refers to any terrifying extended period of time (when terror reigned or was of supreme concern).