what's that software called that makes someone's voice sound like an end boss in World of Warcraft, and is it possible to use it in a screen play? (eg. have it prerecorded, or do something to the microphone)

I don't know what the software would be.

If you're using microphones though you could easily use effects stomp boxes or something similar. A heavy reverb might do it.

It would also be possible for a stageplay. The easiest way would probably be a pre-recording. Just put (V.O. in a booming voice) etc. after the character's name to signify that it needs a Voice Over. Assuming, that is, that it's not the character's normal voice.

With a mike and the proper tech setup, you could even have the actor speak right into the mike.

(But that's for the director of your play to figure out. Don't worry about that right now. If you write it as a stage direction, the tech team will figure out a way to do it.)

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NaNo:
Chronicles of Shydia -- 06'
On the One Path -- 07' - Winner

Script Frenzy:
Stained Glass -- 08'

Agree with others than you can leave this detail for the production team to sort out. But ... I've been wanting to have an actor breathe "anti-helium" to speak a few lines. Surprising (and mostly unknown effect). Check out the clip here: http://www.flixxy.com/anti-helium.htm.