Tenebrous means dark, gloomy, or obscure, and its history traces back through Middle English and Old French to classical Latin roots.  The word derives from the Latin noun tenebrae, which means "darkness" or "shadows".  Tenebrae led to the Latin adjective tenebrosus, meaning "dark" or "full of darkness".  It passed into Old French as tenebros (11th century; Modern French ténébreux) before entering Middle English in the 15th century.  

According to the Oxford English Dictionary (OED), the earliest known use of the adjective tenebrous in English is from around 1420 in the writings of the poet and prior John Lydgate.  

Just the facts:

TENEBROUS Definition & Meaning - Merriam-Webster

TENEBROUS Synonyms: 190 Similar and Opposite Words | Merriam-Webster Thesaurus

"Tenebrous"—meaning dark, shadowy, or obscure—is important in storytelling because it creates deep atmospheric tension, heightens mystery, and mirrors complex moral or emotional struggles.  

Building Atmosphere:  
Sets the mood: Physical darkness makes scenes feel heavy, oppressive, or frightening.  
Engages the senses: Describing shadowy corners or dim hallways helps readers feel isolated and alert.  
Crafts a distinct setting: Gothic castles, deep caves, and foggy alleys use shadow as a living part of the world.  

Deepening Mystery and Suspense:  
Hides vital details: Obscuring what characters can see leaves room for the imagination to fear the unknown.  
Signifies danger: A shadowy environment signals that a threat or ambush might be near.  
Highlights hidden motives: Figurative darkness points to secret deals, lies, or corrupt plots.  

Reflecting Inner Darkness:  
Mirrors psychology: Gloomy settings often match a character's depressed, confused, or troubled thoughts.  
Shows moral ambiguity: A tenebrous plot line features characters who are neither fully good nor fully evil.  
Adds emotional richness: It turns a simple physical setting into a symbol of inner struggle and uncertainty.