immorality vs foul play

immorality and foul play both are nouns.

Word	Noun	Adjective	Verb	Adverb
immorality	Yes	No	No	No
foul play	Yes	No	No	No

As nouns, foul play is a hyponym of immorality; that is, foul play is a word with a more specific, narrower meaning than immorality:

Other hyponyms of immorality include devilry, deviltry, irreverence, violation, sexual immorality.

immorality (noun)	foul play (noun)
morally objectionable behavior	unfair or dishonest behavior (especially involving violence)
the quality of not being in accord with standards of right or good conduct