What finally brought an end to this barbaric reign of King Leopold II?

The reason the reign of King Leopold II ended was that his treatment of the Congolese people caused a major international scandal and the Belgian government forced Leopold to relinquish control.

When the press published the extent of his crimes in the Congo, there was international outrage and the Belgium government then forced Leopold to hand over control of the Congo to them.

In conclusion, the Belgian government ended Leopold's rule.

Find out more on Leopold's atrocities at link.

Philippe-Marie-Victor

Leopold II, French in full Léopold-Louis-Philippe-Marie-Victor, Dutch in full Leopold Lodewijk Filips Maria Victor, (born April 9, 1835, Brussels, Belgium—died December 17, 1909, Laeken), king of the Belgians from 1865 to 1909. Keen on establishing Belgium as an imperial power, he led the first European efforts to develop the Congo River basin, making possible the formation in 1885 of the Congo Free State, annexed in 1908 as the Belgian Congo and now the Democratic Republic of the Congo. Although he played a significant role in the development of the modern Belgian state, he was also responsible for widespread atrocities committed under his rule against his colonial subjects.