intensity vs fierceness

intensity and fierceness both are nouns.

Word	Noun	Adjective	Verb	Adverb
intensity	Yes	No	No	No
fierceness	Yes	No	No	No

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

intensity (noun)	fierceness (noun)
high level or degree; the property of being intense	the property of being wild or turbulent
chromatic purity: freedom from dilution with white and hence vivid in hue
the magnitude of sound (usually in a specified direction)
the amount of energy transmitted (as by acoustic or electromagnetic radiation)