These bells when rung the correct amount of times will activate the Secret Hard Mode.
I do not have every detail on what hard mode changes for the boss, will update when I know more.
If you win this on damnation you get a special insigna!

NUMBER ON IMAGE = HOW MANY TIMES TO HIT THE BELL

This bell is located near spawn. Before you cross the bridge, turn around and you should see it next to the door you came through.Hit it 3 times.

To the right of where the first Twin encounter spawns you will find your second bell. Hit it 5 times.

After the second Karnak encounter, move towards the right staircase and turn around, it is hanging off the pillar there. Hit it 4 times.

After going up the spiral staircase you will find the big chained up wall, turn around and check the left of the slope to find the next bell. Hit it once.

After the ambush fight the Karnak once more then take a left, turn around to find it in the corner. Hit it 3 times.

Once you come to the bazaar enter the alleyway to the right. It is hanging off the wall on your left.
Hit this one a whopping 9 times.

This bell is to the left as you enter the resupply room before the boss. Hit it 7 times.

Youll need it for the fight ahead! Like the guide also thanks :3