If the screen is completely black after the initial scene with the derpy man in a crystal throne, but you can still hear audio, then return to the main menu and go to "Options". Under graphics settings, un-tick Post-Processing.
This setting is not currently functional in the demo for many people (I believe it is people with AMD GPUs), but once you disable it the game should work as intended.
I'm not sure if they'll release a patch for the demo to fix it, but for now, perhaps keep this bumped so people don't need to ask.
*waits until someone says something funny to quote*
I actually played the first couple minutes with a black screen. I saw the quest was something like Out of the Darkness or something, so logic told me, hey, maybe its so dark here I have to use the minimap to find my way out. I somehow looted the sword, and killed the first couple monsters all in complete darkness, lol.
Finally it started telling me to help someone, and at that point I realized there is no way they have you play a game like this for that long. Thankfully it was this easy fix :)