The Nephilim were a minor but malevolent xenos species first encountered on the human-settled world of Melchior by the Blood Angels and the Luna Wolves Space Marine Legions during the Great Crusade in the late 30th Millennium. They fed on the mental emanations of adoration and religious ecstasy and subjugated Melchior's native human population -- and the populations of many other worlds in the region colonised by Mankind -- in order to satisfy this constant psychic hunger.
The Nephilim towered over even Horus, and were twice as tall as an Astartes. They moved slowly, like beasts swimming through water, but could move much faster when in combat. Their native language used hypersonic frequencies and flashes of bio-luminescence, making it impossible to translate for humans. To speak Low Gothic, they utilised a cybernetic implant to generate a vibrating field that created sounds audible to humans; this implant could also be used as a Sonic Weapon.
The Nephilim had round torsos, flat on the back and the front, their heads being little more than lumps atop their torso, with a pair of beady black eyes nestled into their otherwise featureless face. At the end of their long, thick arms the Nephilim had hands with three stubby fingers and their legs were similar in form. The Nephilim social hierarchy was defined by the different colours of their bloated, oblate bodies: blue represented the "rank and file", green the "squad sergeants", and grey the Nephilim's "commanders".
The Nephilim fed on the psychic energy of adulation. They arrived on new worlds and preached a message of spiritual salvation and eventual heavenly reward. Through either psychic manipulation or their sheer alien charisma, the Nephilim converted human populations to their religion. They fashioned masks of their own flesh for their devotees, which were fused to their faces and psychically bonded them together.
In turn, the human faithful built praise-chapels and copper towers to their alien saviours, offering up the adoration their alien masters feasted upon. The Nephilim consumed lives through psychic means, leaving their faithful as desiccated husks bled dry of all vital energies. By the time the Luna Wolves and Blood Angels fought the Nephilim on Melchior, the xenos had stripped multiple worlds in the region of all human life.
Years later, in the early days of the Horus Heresy in 005.M31, the Warmaster Horus tricked his fellow Primarch Sanguinius into taking his Blood Angels Legion to the Signus Cluster, claiming that the Nephilim had not been exterminated, but had returned to enslave the human population of that trinary star system again. Horus also deceived Sanguinius into believing that the Nephilim, with their powers of mental manipulation, possessed a device that could cure the Blood Angels' secret genetic affliction, the Red Thirst. In truth, there were no Nephilim in the Signus Cluster. Instead, the system had been overrun by the daemonic legions of Chaos led by the Keeper of Secrets Kyriss the Perverse and the Bloodthirster Ka'Bandha, who sought to either slay Sanguinius and the Blood Angels, or use their genetic Flaw to turn them to the service of Chaos.
In the Hebrew Bible, the "Nephilim" were the offspring of the "sons of God" and the "daughters of men" before the Deluge, according to Genesis 6:1–4. The word is loosely translated from Hebrew as "giants" in some Bibles and left untranslated in others. The "sons of God" have been interpreted to be fallen angels according to some classical Judaic explanations.
- Fear to Tread (Novel) by James Swallow