Well, anyway, I think I will try the "reverse-verb." I have to do it very lightly (using the platinum verb, 80 dry/20 wet mix, and 1 sec reverb time--that's what STP calls what I believe you mean when you say decay). Too heavy and it makes the words very difficult to understand. I have some ghosts later in the story who just moan. . .maybe I can be more aggressive with it on them.

I'm adding the verb to the pitch and flange FX, and might do a regular reverb on top of the reverse-verb.

This is until I become dissatisfied and change my mind about everything.

I presume recording is already finished?

You can put different takes of the same line on top off each other (the best one the loudest, others pitched or EQed).

If it's not recorded yet: also record whispered versions to put on top of the 'normal' one. Add some reversed reverb and it will sound 'out of this world'. Especially reversed reverb on the beginning of a line: it's sounds like it travels from far.

If you ever need inspiration for evil voices: check myspace and visit: Immortal, Cradle of Filth, Opeth, Deicide, Arch Enemy, Austere.
Not everything I mention is my cup of tea (some might even cause headache), but they al use the human voice to portray evil, anger, torment, etc. Although a lot of the lyrics becomes incomprehensible, it might inspire you on your quest. Use multple layers.

Remember: only downpitching is not evil, it's only Boney M ;-)

Yes, recording is already done.

The multiple takes idea sounds cool, but the reads on each take are generally wildly different--same intent, but different pace, inflection, etc. Without a lot of surgery to sync everything up it was just come out like word noise.

Here's a link that should help you out, enjoy! john.

Designing Sound Jim Stout Special: Making a Scary Voice Effect

Yeah, that's pretty much what I did (except for the delay). I have Logic Express, which doesn't seem to have that "dreamverb" preset (I checked all the five or six flavors of reverb and it's not in any of them). Does anyone know how to recreate it? If not, I'll go with the way I've got it set now.

In the analog.. Oooooooo 'dark ages' (no pun) We use to get the 'devil' effect with the Eventide 910 Harmonizer.. and god-forbid.. flip the tape over for reverse reverb and other out-worldly effects. No quick undo or delete.. Sometimes risking total loss of a song / session / job / career. Can't say I miss those days in some respects.
BTW, the Eventide series are available as plug ins. Not cheap though.

The effect that I find most creepy (even when I am conscious of it) is "chorusing" where the dialog is doubled tripled, quadrupled (etc) with slightly different-sounding timbres of the original speech. It really conjures up that "legion of devils" concept.