Find Alcarnus was the main quest in the Blizzcon 2009 demo. Arcanus was a small, ruined, overrun city deep in the desert wastes, conveniently located at the end of a long, winding dirt road from the gates of Caldeum. Its location may be different in the final game.
Follow the Path
The main quest was given by the NPC Asheara, apparently making her return from Diablo 2. In a short conversation Asheara, who appears ageless and is still employed as a mercenary captain, told the new player that Alcarus was lost, and unreachable across treacherous desert wastes. Your character was undaunted, and replied confidentially that they would find it nevertheless. Each of the four classes had slightly different dialogue, but they were united in their self confidence and determination to see the mission through.
Asheara's dialogue was not customized to the class she addressed, unlike Captain Rumford's in Blizzcon 2008. It's not known if Asheara will be like this in the final game, if her dialogues were not finished yet, or if the developers took out her regular dialogues to avoid giving away plot points in the demo build. She didn't even have a special word for the Barbarian, who she would presumably remember after meeting him twenty years earlier, in the city of Kurast. (In Act 3 of Diablo 2.)
After speaking with Asheara, players had no choice but to work their way out of the narrow, winding canyons into an open area; the Sundered Pass. Through that level, and the levels beyond, a narrow, twisting dirt path led. Following this path would lead characters straight to Alcarus, just as the dirt path led players through all the surface areas of Act One in Diablo 2. It's not known if this path will be there in the final game, or it was just a helpful feature added for the demo.
The Quest
Although the mission to reach Alcarnus was the main quest in the 2009 Blizzcon demo, it was one of the least interesting missions available. There was nothing to do but follow the road through the desert. No clues were offered, no special tricks had to be performed. Your character just had to get there. This was easy to achieve, eventually, but took some hurrying to make it in the 20 minutes players had to try out the game on the show floor. Most players did not, since there were so many fascinating diversions and monsters that led one away from the path.
Completing the quest was also an anticlimax, since there was no end. Players simply had to reach Alcarnus to receive credit for the quest, but there was no reward, or even a final boss battle. The message in the quest window was the only thing that changed, and once you reached the town it simply said, "The scene in the city was even worse than you’d feared."
This was due to the demo being a pared down version of the full game, and the real game will have much more to the quest, plus the city will not be a dead end.
The City
In Alcarnus your character discovered horror and misery. The Dark Cultists had overrun the city and slain or imprisoned all of the inhabitants. Maimed and gruesome corpses were everywhere, many of them skewered on stakes or stretched out on torture racks. Dark Cultists abounded, and had to be slaughtered, ideally before they could transform into the much more dangerous Animated Vessels.
The imprisoned citizens could be freed with a click on their cages, but they did nothing more than thank you before running off and vanishing once they crossed the bridge out of town. Freed citizens would occasionally drop an item, but this seemed to be buggy, as seldom as it occurred.
Other than freeing the citizens and killing the Dark Cultists, there was nothing to do in Alcarnus. The map had been changed for the Blizzcon demo, and the city formed a U-shape, with no exit or way to move through it. It was just a dead end with a curve, and after the cool battle against the Skeleton King that provided a nice conclusion to the 2008 Blizzcon demo, the 2009 Blizzcon version of Alcarnus was a disappointment.