Before the rise of Belah'dia, there existed a nation of black mages in Thanalan. While their history isn't as detailed as the contemporary city-states of Nym and Amdapor, it is known the Mhachi resorted to summoning voidsent as part of their warfare against other nations. Using an elaborate trap, they tricked a Nymian sea galley into bringing an ampoule to their home nation containing a voidsent named Bitoso. This voidsent would unleash a devastating plague called the "Green Death," which transformed many Nymians into tonberries.
It is implied they were the ones to unleash Diabolos on the Amdapori, which would be sealed within the Lost City of Amdapor by its white mages.