Nephilim is the collective name given by Kevin Vacit to various breeds of non-sentient telepaths, specifically engineered by the Vorlons from the genetic material of a hundred different races to create the telepath gene that was then re-inserted into their respective genomes to create viable sentient telepaths.
Since sentient races cannot naturally evolve telepathic abilities and naturally evolved telepathy precludes sentience, the only way for the Vorlons to create the sentient telepathic weapons they desired was to develop the two traits separately and later combine them artificially. The Vorlons went about creating telepaths on a hundred worlds by first visiting a planet with promising yet still primitive life-forms, taking genetic samples or individuals from those races (Homo erectus in the case of Earth), taking them back to their experimental station where they were cloned, bred, manipulated, experimented on and pushed towards an enhanced telepathic ability. The result was a population of docile, non-sentient humanoid creatures with staggeringly powerful telepathic abilities, at a level beyond which even the Vorlons could rate. Their function was to act as hosts for the telepath gene that could be extracted and gradually introduced into their sentient "cousin" races. Like all Vorlon technology, the Nephilim, while incapable of any independent thought or problem solving, were conditioned to obey their creators.
When Kevin Vacit encountered them on the Vorlon experimental station in 2256, he dubbed them the "Nephilim," after the bastard offspring of angels and human women from Hebrew tradition. Around the same time, the Earth Alliance starship Icarus landed on Z'ha'dum, prematurely awakening the Shadows and setting in motion the Second Shadow War before the Vorlons were fully ready. The Vorlons, fearing their "Nephilim" would fall into the hands of the Shadows and satisfied that they had already fulfilled their purpose, bombarded the planet, apparently wiping out any useful traces of their creations.
However, the Vorlons were unaware that Kevin Vacit had landed on the world and with the use of the Vorlon fragment that he had carried with him most of his life, he was able to direct the Nephilim - ever eager to obey what they perceived as a Vorlon - to use their collective ability to fool the Vorlons into thinking they had succeeding in wiping out the millions of Nephilim scattered throughout the forests and savannahs of their world. Though he was able to save only a few thousand, it was more than enough to exert a terrifying amount of telepathic pressure.
In 2265, in the midst of the Telepath War, both Psi Corps and the Telepath Resistance caught up with Vacit on the Vorlon outpost. The Psi Cops, led by Mr. Diamond, arrived first and were instantly overwhelmed by Vacit's Nephilim. When Lyta Alexander and Michael Garibaldi arrived some days later, Vacit used the power to convince them that Vacit and the Psi Cops were dead and that the Vorlons had successfully wiped out the Nephilim and all trace of useful information.
After Alexander and Garibaldi leave, Diamond pleads with Vacit to let them go as well. Vacit tells them the he needs them here and that they still have much work to do.
- The ultimate fate of Vacit and the Nephilim is unknown, though it is clear that the former director of Psi Corps has a plan.