Low pitched effect?

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.

Thanks, will absolutely check it out!
 
That effect totally sound like granulation + pitched down (more like pitched down using granulation algorithm).
If you have FL Studio, it has one of most versatile granulation plugins (fl granulizer), but it's sampler.