onset vs onslaught

onset and onslaught both are nouns.

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

As nouns, onset and onslaught are synonyms defined as:

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

Other synonyms of onset include attack, onrush.

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