The word unholy traces its roots back to Old English, derived by combining the negative prefix un- (not) with the adjective holy. 

In storytelling, the concept of the unholy serves as a vital tool to define moral boundaries, establish visceral dread, and explore the unknown. It represents more than just evil; it signifies a perversion of what is natural, sacred, or whole, acting as the ultimate catalyst for conflict and character growth. 

The narrative importance of the unholy breaks down into several key functions:  
Defining the Sacred: The unholy cannot exist without its counterpart. By introducing elements that are corrupted, profane, or blasphemous, storytellers create a clear contrast that establishes what the characters hold dear—whether that is faith, nature, logic, or humanity. The unholy acts as a threat that forces protagonists to define and defend their core beliefs.  
Visceral Atmosphere and Fear:  In horror, fantasy, and dark fiction, the concept of the unholy is essential for building dread. It frequently manifests as an "unholy nexus" or a corrupted location where the veil between worlds is thin, or where dark rituals are performed. This triggers base human emotions—fear, disgust, and the instinct to survive—which quickly propel the plot forward.  
Subverting the Natural Order: The word "unholy" originates from the Old English word unhal, which means "not whole or healthy". In narrative design, this makes the unholy perfect for portraying entities that disrupt the balance of the universe. Whether it is undead monsters, demonic possession, or a corrupt system of power, these forces disrupt the status quo, demanding resolution. 
Exploring the Shadow Self: Characters who dabble in "unholy" magic, pacts, or corruption often serve as cautionary tales. The unholy externalizes internal struggles such as moral decay, ambition, or unchecked power. Writers use these dark temptations to test a hero's willpower or to humanize tragic villains who have lost their sense of goodness in pursuit of a destructive goal. 
Challenging Moral Certainty: Modern storytellers often use the unholy to subvert traditional expectations of good versus evil. In complex narratives, the sacred itself might be portrayed as flawed or oppressive, while the "unholy" force might be a symptom of a broken society. This forces audiences to grapple with ambiguity and question the nature of objective morality. 

1 hour ago, drpaladin said:

When I hear unholy, my first thought is something lacking holes.

HOLELESS Definition & Meaning - Merriam-Webster

I don't know why, but this made me think of ' Bagels ' , which in turn, led me to ask myself, is a bagel with no hole still a bagel? This, then led  me to the following video(what a small world). Just so you know, I have been to this bakery literally thousands of times in the last 60 plus years.

Is a bagel with no hole still a bagel? Montreal's St-Viateur behind cheeky campaign

Shalom