On the "Reign of Terror" event, do you lose one worker from the worker pool or the population bank?
On page 12 of the rule book, it states the following
When a card says you must "decrease" your population, you return 1 yellow token from your Unused Workers Pool. If you have no Unused Workers, you must remove (and return to your Yellow Bank) a Worker from one of your cards. Do not forget to adjust your indicators.
Therefore, you must lose a worker from your Pool (or one of your cards if no workers in your pool).