Is there any information on how powerful he is? I wouldn't be surprised if he is one of those guys that have a lot of special abilities, but you could still sneak up on him and shoot him with a handgun.
Do you realize that Maar was incredibly powerful? Now, the Fallen One is a Xel'Naga (pretty sure), a species whose nature is that of a hybrid -- having purity of form and essence. I wouldn't be surprised if it was around the power level of Maar (seeing as Maar drew his power from preservers). Prolly he is stronger tho.
I can't find him in the map editor, alas. (If I did, I would have posted them.) He might not be much of a physical threat, anyway, as he's old. This is assuming he's a xel'naga; his artifacts (I'm assuming they're his) are only thousands of years old, rather than millions of years old. Perhaps he's a one-off?
aah yes, because ancient beings forged out of pure shadow are well-known to be frail, meek beings. Especially if their very name evokes power. Guess age does that to a guy. In a more serious manner, he could have simply made the artifacts a few thousand years ago. Or maybe he is the rift in the time-space continuum caused by the decline of the xel'naga and the lack of rise of a new group given form.
I managed to find this: http://imageshack.us/photo/my-images/822/darkvoice.png/sr=1 I want to say dark Voice is a Xel'naga that turned evil and went his own way to pursue plans of his own, seperate from the rest of the Xel'naga race (hence- Fallen one), but the size of this model says otherwise, unless it's either a fake or a mistake. A lit up version of his portrait shows him wearing armor similar in design to a Protoss High Templar (but black), and unless Protoss armor is meant to immitate that of their "gods," I don't know what Dark Voice is.