Hi,
Try this :
http://www.gvst.co.uk/ggrain.htm
Or this :
http://www.solidstatelogic.com/music/X-Orcism/
Might do the trick.
If you want to do it yourself, you can try to duplicate your main vocal track several times, and pitch down each dup with a different amount (for example, one track -3, another -5...etc and blend them together for a devilish effect), play with panning if needed, then add some distortion, chorus...etc.