It happened in 1909 in the Congo Free State, which case was the first trial between a multinational company and human rights activists?

The Congo Free State (1885-1908) was a Central African absolute monarchy privately owned and controlled by King Leopold II of Belgium. The King had seized the region which became a separate nation.

The area was treated as a source of raw materials, including ivory, rubber and minerals. This was at the cost of atrocities and loss of life inflicted on the local people via forced labour, torture and murder. Some estimates quantify the number of deaths as high as 50% of the population.

Missionaries carefully documented and exposed atrocities committed. Eye-witness reports from missionaries portrayed actions by the State that broke laws set by the European nations. As rumours circulated Leopold attempted to discredit them, even creating a Commission for the Protection of the Natives. In January 1908, William Henry Sheppard published a report on colonial abuses in the American Presbyterian Congo Mission (APCM) newsletter, and both he and William Morrison were sued for libel against the Kasai Rubber Company (Compagnie de Kasai), a prominent Belgian rubber contractor in the area.

https://wiki2.org/en/Congo_Free_State+Newton

The 1909 court case against the missionaries (who were supported by the Congo Reform Association) saw them acquitted.