CAGED IN STONE
The Royal Palace is located in Orzammar's Diamond Quarter. Within the empty throne room of the palace, the king's throne is located and it has an information point on it. Looking at this will give the Warden the Codex entry: Caged in Stone.
You may have noticed that while walking within the Palace there are areas that will create a shifting gears sound. There are three of these pressure plates:
Just outside the throne room. It is the large decorated square tile.
Inside the throne room. Facing the throne, look to the left for an exposed column of lava. The tiles at the base look like an arrow pointing into the lava. The two pressure plates are the "tails" of the arrow.
When a character steps on a plate, it will make a loud clicking sound. Position one companion (or a pet/summon) on each plate (place party on hold), then with selected character click on the throne again. If you have done this correctly, there will be lightning and a dragon will appear, which you have to slay.
Note: Once the dragon appears the person (or pet) outside the throne room is teleported inside the room and both doors lock. A rogue can unlock them (for 5 XP each) but no one else joins the battle.
Rewards
When you kill the dragon, it drops:
Ageless (Silverite Greatsword)
Heavy Chainmail (Varies, not guaranteed)
Note: This dragon does not drop a Dragon Scale.