unknown vs unheard-of

unknown is a noun but unheard-of is not a noun.

Word	Noun	Adjective	Verb	Adverb
unknown	Yes	Yes	No	No
unheard-of	No	Yes	No	No

unknown (noun)	unheard-of (noun)
a variable whose values are solutions of an equation
an unknown and unexplored region
anyone who does not belong in the environment in which they are found

unknown (adjective)	unheard-of (adjective)
not known	previously unknown
being or having an unknown or unnamed source
not famous or acclaimed
not known before
not known to exist