"The Devil in the Details" is episode fourteen of season five of Bones, the forensic drama television series loosely based on the novels and life of anthropologist Kathy Reichs. In this episode, a body with horns and a tail is found burning on an altar.
The Burning Horned Body on the Altar
A priest and an altar boy are getting ready for Mass when the boy comments that he can smell something. The priest says it's the new candles; they're more smoky than the previous ones. On entering the main part of the church, the smell is revealed to be coming from something different - a body on fire on the altar. When approached, the priest starts saying prayers, for the burning body has horns.
Booth is Concerned
Booth is concerned about taking Brennan to a Catholic church, given her opinions on the subject. Such as Brennan making comments that Zeus could also throw thunderbolts and seemed to have far better aim with them than Booth's God. Booth is also concerned about a body with horns. Especially as Brennan says that the horns are actually made of bone.
A Symbolic, Or Perhaps Worrying, Body
At the Jeffersonian, more religious discourse occurs - it's also Mr Vaziri's turn as intern (he has dropped his fake accent after what happened in "The Beautiful Day in the Neighborhood"). The body also has a vestigial tale. A horned, tailed body aflame on the altar of a church seems rather symbolic. Or worrying, depending on your point of view. As well as repeated discussions on the nature of good and evil - Brennan doesn't really believe in evil, just things like poor role models and imbalances.
Not a Devil
The tail is real - vestigial - but the horns are actually implants. The body belongs to a man, Neil, who was being treated for schizophrenia at a mental institution, but who had left without permission (he had gone missing before, but usually within the institution). He had the delusion that he was given his tail by Satan, and implanted the horns as part of this.
Brennan is Not Suited to a Psychiatric Institution
Booth and Brennan go to the institution, accompanied by Sweets. One of the patients there - who believes she's the angel Uriel - has painted a picture showing herself stabbing the crucified victim in a place where he was injured. Brennan is definitely not the best person to interact with people who think that they are angels. She is also not the best person to deal with psychiatrists, given her disdain for the profession.
It still isn't clear how Neil died. Or why.