The show centers on FBI special agents who work on cases linked to the paranormal, called X-Files; this season focuses on the investigations of John Doggett (Robert Patrick), Monica Reyes (Annabeth Gish), and Dana Scully (Gillian Anderson).

When creating Josef Kobold, the episode's antagonist, Spotnitz researched various crimes and criminals, until he learned about Caryl Chessman, a man who had been sentenced to death on a technical charge of kidnapping.

Series co-star Robert Patrick had issues remembering and delivering his lines, due to the theme of demonic possession, which reportedly made him uneasy.

Following a bizarre double-murder with Satanic ritual overtones in Weston, West Virginia, John Doggett (Robert Patrick) and Monica Reyes (Annabeth Gish) are offered the case.

Dr. Monique Sampson calls them, saying that the murders may be connected to an escaped mental patient, Dr. Kenneth Richman, and a guard, Paul Gerlach.

After putting Kobold in the custody of another guard named Custer, Doggett and Reyes race to Sampson's home and find her dead, with a dozen hypodermic needles jammed into her face.

That night, the power suddenly goes out in the mental institution: Custer approaches Kobold's cell and witnesses him turning into a demon.

Explaining that the word "Dae/moni/cus" was code for the chosen victims' names, in a game Kobold devised to culminate in his eventual escape from the asylum.

"[3] When creating Kobold, Spotnitz researched various crimes and criminals, until he came across the name of Caryl Chessman, who had been sentenced to death on a technical charge of kidnapping.

[3] Series co-star Robert Patrick had issues remembering and delivering his lines due to the theme of demonic possession, which reportedly "was freaking" him out.

[9] Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, rated the episode one-and-a-half stars out of five.

The two criticized Spotnitz "attention seeking direction", citing the "strange cross fades" and "exaggerated camera angles" as detractors.

[11] He also felt that, after "Nothing Important Happened Today"—which he called a "snooze-fest"—"Daemonicus" was a "disheartening [way] to get back to monster of the week episodes".

Crang, in his book Denying the Truth: Revisiting The X-Files after 9/11, said the episode was "sluggishly paced" but praised the "wonderfully tense atmosphere" and Spotnitz's "stylish directing choices".

[12] In a season review, Michelle Kung from Entertainment Weekly called the episode "worthy", but noted that it was overshadowed by the show's "ludicrous conspiracy plots".