Desolate dive is a great spell/ attack that can be used to open up different paths and can be upgraded to Descending Dark. Desolate dive is found in the City of Tears and this guide is going to show you exactly where to go step by step.
Requirements for Desolate dive:
We are going to start from here in the Fungal Wastes. This is a place that you had to go to earlier in order to get the Mantis Claw.
In the next room there is a bunch of acid at the bottom and you will need to use the Mantis Claw to wall jump and platform your way through. Not going to lie, this is one of the harder areas of platforming until much later in the game.
Once you make it to the end of the bridge there is a statue that will ask you to place the City Crest item here. You should have received the City Crest from beating the first boss known as the False Knight.
Now that you are inside the City of Tears, make sure not to leave until you have found a stag station. The city crest can only be used once, and you are essentially locked in at that point. The door you just came through can’t be used again. There are other ways into the city but this is by far the easiest.
Now is it just a matter of following the only path available until you make your way to the ground floor of the City of Tears.
Note if you go to the left this is where you will find the Nail Smith. This NPC can upgrade your nail from an Old Nail to a Sharpened Nail for only 250 geo. To upgrade your nail again, you will need to find Pale Ore and pay more geo.
After upgrading your nail head back to this same elevator that got you down to the bottom floor. From here, ride the elevator up just a bit and jump to the right on this platform.
Continue to the right until you get into a new room. This room is large and will require you to climb to the top. From the top you have the option to go to the left and rest at a much-needed bench. Just follow the signs.
You will have to fight some enemies before sitting at the bench. This bench is going to be the last place you get to save before a boss fight. It is a very long way from the bench to the boss and it is going to be a tough fight.
Make your way back down from the bench and to the right of the elevator that you climbed earlier.
The next room is blocked by an enemy known at the Soul Twister. You will need to defeat this enemy once and then the path will continue to be unlocked even if you die.
The rest of the way to the boss fight is very straight forward. Just follow the path around to where the red arrow is.
This room will have a boss known as the Soul Master. Just stay calm and avoid his attacks. Just know that there is a second phase that happens after you think you think you won. This is a tough fight but you will be able to get it.
After defeating the Soul Master you will earn the Desolate Dive ability. This can be used to open paths throughout Hollow Knight. From here I would suggest going to Crystal Peak and going after the Crystal Heart ability.