Yes, ONSLAUGHT is a Scrabble word!

Letter values in Scrabble US: O 1, N 1, S 1, L 1, A 1, U 1, G 2, H 4, T 1.

Is it a valid word?

ONSLAUGHT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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