The Nephilim were half-angel, half-human quadruplet girls, fathered by an angel to a human woman. They were mentally challenged and presented deformities, such as polydactyly and wing-like protuberances on the shoulders. Their mother having died in childbirth, they were scattered around the city of Alexandria, Virginia, one having been adopted by a Catholic couple, one living in a mental institution, one living in the streets and one hidden by Father Gregory, a caring young priest who meant to protect them.
The girls' souls were coveted by Satan, disguised as social worker Aaron Starkey, but angels came to claim them before he did. The girls died in a kneeling position, their eyeballs burned out from laying eyes on the bright countenance of the heavenly beings. The sisters' strange deaths were initially misunderstood by Father Gregory and by FBI agents Mulder and Scully, who investigated the case. Scully was granted to see one of the angels who had four faces; the face of a man, a lion, an eagle and a bull. (TXF: "All Souls")
A Bible passage (Genesis 6:4) referring to the Nephilim being the sons of God's offspring and another (2 Samuel 21:20) describing a descendant of the Nephilim having six fingers on each hand and six toes on each feet inspired Billy Brown and Dan Angel, who wrote the story of "All Souls", to create the episode's half-angel, half-human characters. Choosing the supernatural interpretation of Genesis 6:4's "sons of God" to mean angels, they imagined the four polydactyl sisters to be born of a union that was not meant to be, between an angel and a human woman. The idea of one of the angels having four faces – those of a man, an eagle, a lion and a bull – is exactly how a group of four biblical Cherubim are described in Ezekiel 1.