Based on the classic gothic horror novel by master of the genre the late James Herbert.

The publicity/marketing campaign for "The Unholy" utilized preeminent Parapsychologist Christopher Chacon to promote its release. As one of the world's foremost authorities on supernatural/occult and metaphysical phenomena, Chacon has investigated and researched thousands of cases around the world of miracles and divine-intervention phenomena, as well as thousands of possessions and supernatural infestations, having worked with the Roman Catholic Church as well as every religious/spiritual belief-system.

Katie Aselton replaced Jordana Brewster due to scheduling conflicts.

The story line was base on James Herbert's Clarice novel 'The Shrine'

In Spain the movie was titled "Ruega por nosotros" (Pray for us), referencing the "Ave María" ("Hail Mary" in English), a Christian prayer about the Virgin Mary: -Spanish: "Dios te salve María, llena eres de gracia. El Señor es contigo. Bendita tú eres entre todas las mujeres, y bendito es el fruto de tu vientre, Jesús. Santa María, Madre de Dios, ruega por nosotros pecadores, ahora y en la hora de nuestra muerte. Amén". -English: "Hail Mary, full of grace. The Lord is with thee. Blessed art thou amongst women, and blessed is the fruit of thy womb, Jesus. Holy Mary, Mother of God, pray for us sinners, now and at the hour of our death. Amen".