From Diablo Wiki
He's not a regular employee of Blizzard.
Michael has been interviewed on several occasions, about his voice work in general as well as specifically on the Diablo series. Here are a couple of quotes from an interview with Mtv in September, 2008. 
- On how the Sean Connery-esque voice of Diablo‘s Deckard Cain came about:
- Gough: Deckard Cain has a little bit of that Sean Connery thing that he does. I don’t remember exactly how it came about but I believe that the creators, the guys from Blizzard, threw the Sean Connery name out there. He was supposed to sound older and wiser but have a little bit of a kick to him, a little bit ornery but in that kind of sarcastic Sean Connery way. He doesn’t take a lot of BS, if you will. He’s a very fun character. And sometimes he’s kind of dark and doom and gloom, with all the “evil that spreads over this land will never be abated” but then other times when he gets cranky, it’s funny. “No one ever listens!” And I don’t know if you’ve ever heard the Deckard Cain rap, but I just started letting it go, and I have to boast I did make up a good portion of the lyrics. [laughs]
- On how much Deckard Cain is in Diablo III
- Gough: Well, I did more than [what was shown at the Blizzard Worldwide Invitational in Paris]. I hope there’s more, but I did a pretty fair amount. It was just one or two four-hour sessions but there was a lot. It was definitely more than what you’ve seen in the preview. I’m not sure when it’s release date is targeted for, but yeah it seems like there’s going to be a fair amount of Deckard Cain, which is always a good thing. [laughs]
Another audio interview with PurePandemonium from August 2008 has some interesting discussion. Plus it's odd to hear him use his real voice, which is nothing like Deckard Cain's, as heard in the Diablo games.
- Michael Gough the voice actor is sometimes confused with the English stage/screen actor of the same name. The 'other' Michael Gough is probably best known to Diablo's demographic as Alfred the Butler in the Batman movies of the 80's and 90's.
- Gough's Website has dozens of samples of his voice work, but the only sample of Diablo is the Cain rap which doesn't actually appear in-game.
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