Quote Originally Posted by beholder
unless i missed it in the book, i'm pretty sure the screams were just peter jackson's adaption of the black breath
Nope - the scream is mentioned explicitly just before the hobbits get to Farmer Maggot's in Shortcut to Mushrooms - one cries out and another, far off, answers. Frodo says "It was a call or signal - there were words in that cry though I could not catch them." That would support making it a communication aid, and a fear attack (the hobbits hearing the cry are startled and shaken more than as if it were just a sound...)
The Black Breath is what takes down Merry when he goes wandering out for a walk at the Pony in Strider.

Also, I'm thinking that if one gets a save against the Nazgul's touch attack, it should be higher damage - maybe 2d6 + CHA modifier?

Otherwise - splendid work! [applause!]