Nearing the end of the Khrumbul-Dun mission you will need to find diamonds in order to be able to create an item to help you beat the world boss. Diamonds will be the toughest thing you’ve had to find yet, as they are deep within a lava-filled cave system. Getting into the cave system is fairly easy as Goldirox will show you the way, but you’ll quickly find that there are plenty of enemies and the lava floor to contend with.
You will, however, use the Echo Flute to try and track them down. To make things a little bit easier you’re going to want to know how to cool off the lava filled floors so you can pass. You can make it from checkpoint to checkpoint by taking damage from the lava, but it’s probably easier to just use your bottomless jug of water to cool the lava off as you progress. While you can’t pour the water on the lava directly, you need to find a non-lava spot and let it overflow into the lava to cool it off.
Once you do that, you’ll still need to contend with the many enemies in the area, but it does make things easier. The cave system is fairly linear, but it does have some tricks up its sleeve. You’ll encounter two naviglobes along the way, so make sure you interact with those so you can fast travel back if you happen to die.
The second naviglobe spot is right next to a the Sinister Shrine where you’ll encounter a bunch of enemies and have a dialogue with Malroth. If you make it here, you are very close to the diamonds which are hidden behind a lava waterfall. Play the Flute at the area in the images above and you will see the notes coming from behind the lava.
The easiest way to get here is to just jump through the lava as there is a clearing on the other side, and a cavern with the 10 diamonds that you need to progress.