As part of the skeleton / undead area of the map, you will come across an island that contains three large bells and a sign post where the text is missing. The solution isn’t as hard as it might seem, but it also seems to be a bit glitched. I figured this one out and entered the pattern and the chest did not unlock for me. I later found out that I was doing it correctly but the game wasn’t unlocking it for some reason.
If you enter the sequence wrong, I am unsure how you reset it and this may have been the cause of the problem. I can only guess that reading the sign again will reset it for you and you can attempt to enter it again.
How to solve the giant bell puzzle
The sign post actually gives away the order but for some reason its backwards. I would have read the top line as being 1 and bottom as 3 but the game does it the other way around. You will need to hit the bells in this order in order to unlock the chest.
Here is the order that you need to ring each of the bells. To be safe, press the button that is given in the prompt to interact with the bell. They also ring when you hit them but I wasn’t able to get this to work. It only worked when I interacted with the bells to ring them.
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Quitting the game and going back in will reset the puzzle. As for the order, it is not backwards, it is representing the pitch of the note played. The right most bell is the higher note and therefore higher in the sequence.