Short answers, pulled from the story.
King Leopold II of Belgium owned the Congo Free State as his private property from 1885 to 1908. He controlled the territory from his palace in Brussels without ever visiting the region. The state was legally defined as the Domaine Privé, the private estate of a single monarch.
The Congo Free State existed from 1885 to 1908. The Belgian government annexed the territory as a colony on the 15th of November 1908. King Leopold II died in Brussels on the 17th of December 1909 after losing his absolute power.
King Leopold II controlled the Congo Free State through the International Association of the Congo and the Force Publique. He used Henry Morton Stanley to secure treaties with local chiefs and established military posts between 1879 and 1884. The state enforced rubber quotas through a private army armed with modern weapons and the chicotte bull whip.
The population decline in the Congo Free State was caused by indiscriminate war, starvation, reduction of births, and disease. Sleeping sickness was a major cause of fatality during the period. Estimates suggest that roughly half the population perished, with some historians calculating a total of 10 million deaths.
William Henry Sheppard, an African American missionary, documented cases of cruelty in the American Presbyterian Congo Mission newsletter. Roger Casement, the British Consul at Boma, issued the 1904 report known as the Casement Report. E. D. Morel founded the Congo Reform Association to end Leopold's control and included figures like Mark Twain and Sir Arthur Conan Doyle.