darkness vs foulness

darkness and foulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
darkness	Yes	No	No	No
foulness	Yes	No	No	No

As nouns, foulness is a hyponym of darkness; that is, foulness is a word with a more specific, narrower meaning than darkness:

darkness (noun)	foulness (noun)
a swarthy complexion	the attribute of having a strong offensive smell
having a dark or somber color	(of weather) the badness of the weather
an unenlightened state	a state characterized by foul or disgusting dirt and refuse
an unilluminated area	disgusting wickedness and immorality
absence of light or illumination
absence of moral or spiritual values