onslaught vs onset

onslaught and onset both are nouns.

Word	Noun	Adjective	Verb	Adverb
onslaught	Yes	No	No	No
onset	Yes	No	No	No

As nouns, onslaught and onset are synonyms defined as:

onslaught and onset: (military) an offensive against an enemy (using weapons)

Other synonyms of onslaught include attack, onrush.

onslaught (noun)	onset (noun)
(military) an offensive against an enemy (using weapons)	(military) an offensive against an enemy (using weapons)
the rapid and continuous delivery of linguistic communication (spoken or written)	the beginning or early stages
a sudden and severe onset of trouble