You have to go up. Sprint towards stairs when the ball bypasses you.
When you get to the middle of the stairs, you'll see a foggy passage to the left. Hide there.
Before you enter fog, you might run up to the chamber guarded by a mage.
In the chamber you'll find a chest with Ring of Steel Protection.
Go back to fog and enter it. The first chamber is empty but in the next you'll find lurking lizard. Be careful, because there is a button in the floor between chambers, which activates arrow trap. You can use it to kill a lizard (just lure him out and flee - he will activate a trap).
A bit further you'll get to the staircase, with the same giant ball as previously, rolling down.
In this situation it is best to do like this: wait until ball passes you and quickly run the stairs up. Keep running until you reach a place with damaged column and then jump down next to it. You'll get to the lower platform guarded by a lizard mage.
Now enter the corridor with stairs. At its end you'll find Black Sorcerer set and spell Hush.
Now open the cell above and enter it. Then go to the left.
You'll see a ramp to a chamber, where giant rocks are loaded. You'll come back here soon. Now move forwards.
Move down the elevator, but only when it goes down, because there are spikes at the top!
At the bottom you'll see a chest - DO NOT OPEN IT. It's a Mimic, monster. Attack the chest and fight it. His blows are very powerful, but he is slow. Surround him and block his attacks. For killing mimic you'll get Lighting Spear - very good weapon.
Go to the corridor with balls and turn left. Wait until balls fill a hole. The next rock will destroy the wall.
On the other side collect Covetous Gold Serpent Ring.
Now return to the place, which leads to a chamber where rock balls start their "journey" (use the elevator to go up).