Short answers, pulled from the story.
The Coup of 18 Brumaire was the seizure of power by Napoleon Bonaparte that ended the French Revolution. It took place on the 9th of November 1799, which corresponded to 18 Brumaire, Year VIII, under the French Republican calendar. The coup overthrew the Directory and replaced it with the French Consulate, with Napoleon as First Consul.
Emmanuel Joseph Sieyès was a member of the five-man ruling Directory and the primary architect of the conspiracy. He became the dominant figure in government after the Coup of 30 Prairial VII on the 18th of June 1799, and recruited Napoleon as the military figure his planned coup required. Napoleon ultimately outmaneuvered Sieyès and took power for himself rather than serving as Sieyès's instrument.
Napoleon was met with heckling, jostling, and outright physical assault when he entered the Council of Five Hundred on the 19th of Brumaire. A motion was raised to declare him an outlaw. His brother Lucien Bonaparte, who presided over the council, called in grenadiers under General Joachim Murat, who marched into the Orangerie and dispersed the deputies by force.
The Constitution of the Year VIII was drafted by commissions assembled after the coup and was described as short and obscure. It was the first French constitution since the Revolution without a Declaration of Rights. It gave the First Consul greater power than the other two consuls; Napoleon controlled Senate appointments, and the Sénat conservateur allowed him to rule by decree, relegating the Conseil d'État and Tribunat to minor roles.
In 1852, Karl Marx wrote The Eighteenth Brumaire of Louis Bonaparte in response to the 1851 coup by Napoleon III, who was Napoleon's nephew. Marx opened the essay with his oft-quoted observation that Hegel had noted great world-historic events appear twice, but forgot to add: the first time as tragedy, the second time as farce. Marx considered Napoleon III a trifling figure compared to his uncle.
Lucien Bonaparte, as president of the Council of Five Hundred, intervened at the critical moment when Napoleon was being physically assaulted and a motion to declare him an outlaw was being raised. Lucien exited the chamber and told soldiers that deputies were terrorizing the majority with daggers, pointing to Napoleon's bloodied face as proof. He then drew a sword and theatrically swore to kill his own brother if he proved a traitor, which prompted the troops to disperse the council.