Damon Dash -- Ex-Wife Pulled a Knife On Me and Jacked $20k Worth of Bling

Damon Dash's not about to give up custody of his daughters to an ex-wife he claims drove drunk with one of the girls, stole diamonds from him, and threatened to cut him with a knife.

Jay Z's ex Roc-a-Fella partner filed docs demanding full custody of 15-year-old Ava and 6-year-old Tallulah -- and laid out some terrifying allegations about the girls' mother Rachel Roy.

In the docs, Dash says Roy threatend his life on several occasions -- once by throwing plates at him, and also by pulling a knife on him. He says sometimes the threats came in front of little Tallulah.

Dash paints a "Mommie Dearest" image of Roy -- saying he witnessed her driving drunk with Tallulah in the car -- and adds he's heard heated arguments between Ava and her mom ... where they call each other "bitch" and get physical.

Roy's also one helluva a jewel thief, according to Dash. In the docs, he says she stole 2 diamond chains -- worth $10k each -- and a suitcase. Not your Samsonite either ... it's a Bottega Veneta worth $5k!

And Dash says he's been blacklisted at Tallulah's fancy L.A. private school, Buckley ... because Roy told the school he's not allowed to see or pick up their daughter.