This is a story related quest. One of the first quests you'll be given is to find Colbert in the city of Amaranthine and speak to him. He's right near the entrance to the city on the right side. Talk to him and he'll tell you about the chasm. Travel to the new location on your map (Knotwood Hills) and enter the city of Kal-Hirol. At the entrance to the city, you'll have the choice of going the front way or the side way. The side way is much easier. When you're facing the steps, on the left side there will be a barrier that goes to a wall. On the wall, there is a large triangle with faces on either side of the triangle. The left face has a switch for the secret entrance. After fighting through legions of Darkspawn (and meeting the Children), you'll come to a room where you'll fight the Inferno Golem and The Lost. The Lost is like any mage you've faced before, only with more hitpoints. Same goes with the Inferno Golem (frost runes work nicely on it). After defeating both, you'll enter a room with a pit in the middle and chains at each corner. Fight through the broodmother tentacles and break the chains. I only had to break two of them (both on the same side), but that may vary from game to game and difficulty to difficulty.