Yes, DOWNFALL is a Scrabble word!

Letter values in Scrabble US: D 2, O 1, W 4, N 1, F 4, A 1, L 1, L 1.

Is it a valid word?

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

failure that results in a loss of position or reputation
the falling to earth of any form of water (rain or snow or hail or sleet or mist)
a sudden decline in strength or number or importance