Sneak Past the Ballistae is a side quest in The Darkspawn Chronicles DLC for Dragon Age: Origins. It is available as part of the main quest Massacre in the Market. Take control of a Shriek and use it to sneak past the ballistae set up to block off the Market District.
Just inside the gates there will be a shriek to your left. Cast Enthrall on it to add it to your party. It should automatically cast Stealth on itself, but if it doesn't make sure to cast it. Select "hold" to have all other party members stay in place, then move forward to the three ballistae blocking off the Market District with the three guards waiting behind them. Squeeze in between the ballistae and attack one of the guards standing there. This will cause you to unstealth, so make sure to kill the rest of the guards quickly. Once they've all been killed, command the party to "move freely" and proceed into the district.
You now have access to the Marketplace, and the next quest Kill Bann Teagan will trigger.