Nephilim

The Nephilim were a minor but malevolent xenos species encountered on Melchior by the Blood Angels and the Luna Wolves during the Great Crusade. They fed on adoration and worship and subjugated their world's native human population in order to satisfy this hunger.

History
The Nephilim towered over even Horus, up to twice as tall as a Legionary, and moved slowly, like beasts swimming through water, but could move much faster when fighting. Their native language used hypersonic frequencies and flashes of bio-luminescence, making it impossible to translate. To speak Low Gothic, they utilised an implant to generate a vibrating field to create sounds; this implant could also be used as a sonic weapon. Their hands had three stubby fingers. Their social hierarchy consisted of distinguishing colours of their bloated, oblate bodies: blue "rank and file", green "squad sergeants", and grey "commanders".

The Nephilim fed on adulation. They arrived on worlds and preached of heaven and salvation. Through either psychic manipulation or sheer 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 faithful built praise-chapels and copper towers. The Nephilim consumed lives, leaving their faithful as desiccated husks. By the time the Luna Wolves and Blood Angels fought the Nephilim on Melchior, the xenos had stripped worlds of all human life.

The Nephilim's homeworld was eventually located by the White Scars, and Jaghatai Khan oversaw the destruction of the Nephilim.

Horus Heresy
Years later, during the start of the Horus Heresy, Horus tricked Sanguinius into taking his Legion to the Signus Cluster, claiming that the Nephilim had not been exterminated, but had returned to enslave the human population again. Horus also deceived Sanguinius into believing that the Nephilim, with their experience of mind manipulation, possessed a device that could cure the Blood Angels' secret affliction.