Children of the Yellow Lake

From the introduction:

This scenario takes place in the city of Manaus, in the Brazilian state of Amazonas, during the year of 1922. The investigators will meet a group of Hastur worshippers that have discovered a special plant from the rainforest that can induce people to dread dreams with He Who Is Not to be Named. Better than that, this plant is strongly addictive - so, with a good distribution system, the cultists believe that they'll be able to enslave all mankind. As the search goes on, the investigators will have an opportunity to face the King in Yellow and, perhaps, the Unspeakable himself.