croak vs succumb

croak is a noun but succumb is not a noun.

Word	Noun	Adjective	Verb	Adverb
croak	Yes	No	Yes	No
succumb	No	No	Yes	No

As verbs, succumb is a hyponym of croak; that is, succumb is a word with a more specific, narrower meaning than croak:

croak: pass from physical life and lose all bodily attributes and functions necessary to sustain life
succumb: be fatally overwhelmed

Other hyponyms of croak include abort, asphyxiate, stifle, suffocate, buy it, pip out, drown, predecease, famish, starve, fall, yield.

croak (noun)	succumb (noun)
a harsh hoarse utterance (as of a frog)

croak (verb)	succumb (verb)
pass from physical life and lose all bodily attributes and functions necessary to sustain life	consent reluctantly
make complaining remarks or noises under one's breath	be fatally overwhelmed
utter a hoarse sound, like a raven