Nyklos was a Human born on one of Apophis' mining planets who attempted to rebel against their 'god' unsuccessfully. Nyklos himself was among a group of likewise-minded individuals who attacked both Apophis and Marteen when they came to assess a new naquadah mine. Although the rest of the group were slaughtered by Apophis' Serpent Guards, Nyklos lived and was brought before Apophis to be tortured. Nyklos spat at his 'god' and cursed his name even as Apophis brought Nyklos to his knees with a ribbon device; a fact which impressed Marteen, a secret Tok'ra spy.
This would have been the end for Nyklos if the rebel group hadn't managed to fatally wound Marteen's host; instead of letting Apophis kill Nyklos, Marteen instead asked his 'master' to allow him to blend with the human. This suitably amused Apophis, who liked the idea of Nyklos being forced to serve the Goa'uld against his will and so Marteen's symbiote entered Nyklos. Although Nyklos fought against Marteen, he soon discovered that Marteen hated the Goa'uld just as much as he did and instead allowed the blending to complete.
Nyklos was now empowered to fight against not only Apophis, but also all other Goa'uld in a way he simply was not able to before. Over the years, the combination of their skills proved to be an effective tool in the Tok'ra's fight agains the Goa'uld. (RPG: "Living Gods: Stargate System Lords")