In the courtyard hub of the first level, Cair Paravel, head to the south end of the courtyard, up the set of steps to your left and stand on the pressure plate. This opens the treacherous looking gate to your right...so go in.
In the next area, go down the hallway, up the stairs to your right and inflict deadly violence on the Telmarine soldier you find there. Go on up the rest of the way to find two more Telmarines that you can kill to death.
Go left and down to come out on a balcony. Savagely slaughter the Telmarines you find there and then use Bacchus (or whatever the satyr-looking character's name is) to shoot the four chains holding up the chandelier. (LB and A or X). This dastardly ploy will drop the chandelier on the heads of some extremely unfortunate Telmarines in the room below.
After the cutscene, unleash your animal instincts and inflict fatal injuries on all the Telmarines in the room. You need to do this before they open the chest at the top of the room, which is conveniently monitored for you by a handy timer at the top of the screen.
Once the room is littered with lifeless Telmarine corpses, go back behind the row of thrones (how many Kings and Queens does this place have?) and grab the...thingy. Place it in the other thingy on the left. Return behind the thrones and pick up the weight and put it on the pressure plate on the right. I'm not really sure what this mysterious contraption does, but you can now go open the chest in front of the thrones and retrieve the stupid horn that you just risked life and limb for.
All that remains is a cutscene and the unlocking of your well-earned 10GS.