The first thing you will need to do is reach this point as shown below and a little cut scene will start. After that, you have to fight the monster and just kill him.
Now before you go into the mine, you have to head over to the left and grab the chest there.
After that, pull the lever of a giant machine and it will cause a big block to move over. You have to go to the top of the block and freeze the water.
It will make the chunk of rock go back to the other side and you will find the legendary chest on the other side.