The Synthesists were a group (commonly labelled a cult) that arose on Illium during the Reaper War. Synthesists held that the Reapers were the natural evolution of life, and believed compliance with them would be rewarded with entry to a superior state of being.

Having much in common with other Reaper-worship groups and cults that arose during the War, the Synthesists of Illium first publicly declared themselves, and appeared primarily based, in Nos Varda, the city to which the survivors of Nos Astra evacuated. Early appearances consisted of posts on public message boards and discussion forums suggesting that the Reapers were misrepresented by those who feared them or failed to understand their true purpose. Although many of these initial arguments and tracts were posted anonymously, the repetition of concepts and phrases (and often entire sections of text) quickly identified them as belonging to a shared source.

Although initially spurring some debate concerning the Reapers' purpose and nature (though almost invariably denouncing the notion that, whatever they were, they could be characterised as desirable for organic life), the Synthesist movement quickly came to be seen as a disruptive element, and potential threat. When confirmed evidence of the indoctrination effect of Reapers began to be widely disseminated, the Synthesists were immediately suspected (although - while this was the case with other Reaper-worship groups, both on Illium and elsewhere - no solid evidence of indoctrination among Synthesist authority figures was ever produced).

The movement also seemed to lack a strong hierarchy, existing in a disorganised sprawl of individuals and groups self-identifying as Synthesists in spite of differing views on the movement's beliefs and goals. While peaceful Synthesists did exist, the movement - by now seen almost exclusively by outsiders as a cult - also became attached to a number of unstable individuals and sub-factions who advocated, and in some cases attempted, the sabotage of Nos Varda's defenses to allow the Reapers to capture the city and "uplift" its population via conversion to husks. The "Synthesist cult" had already been declared in breach of Illium law by the Nos Varda Provisional Authority due to indoctrination concerns; when hostile rhetoric and the advocation of violent action emerged, the Nos Varda Police Department and Eclipse peacekeepers were given broad authority to suppress the movement. Although a number of acts of minor sabotage, and one notable riot, were orchestrated, their effect on the city's defensive capabilities was ultimately marginal.