Quick answer: The Cave Prison Key drops from the Flame-Crazed Bandit and Hound Trainer in the pit under the cave prison below the Thieves’ Camp. It opens a locked cell above the pit that holds a Sha-Chi Amber Shard.
The key is a Story Item in the Crimson Forest, and you need the Thieves’ Camp Dungeon Key first to get into the cave where it drops.
How to reach the cave prison
The cave prison sits in the Crimson Forest, beneath the Thieves’ Camp. The pit where the key drops is roughly 78 m from the camp bell and about 13 m below it.
Reach the entrance through a barred wooden door set in a rocky passage beneath the camp. To unlock it, carry the Thieves’ Camp Dungeon Key, approach the door, and press E.
In our run, the walk went from the Thieves’ Camp bell through the camp and down a slope to the barred door, then on through the tunnel to a wooden prison with cells built around a deep pit.
In our run, a Firethorn Thief in the camp threw firebombs from range on the way through.
How to get the Cave Prison Key
The key doesn’t lie on the ground; enemies down in the pit beneath the prison drop it. They are a Flame-Crazed Bandit and a Hound Trainer, accompanied by Frenzied Hounds. None of them move until you reach the pit.
A wooden ladder joins the pit to the prison floor above. In our run, Soul stepped down into the pit beside the ladder, and the ladder was the way back up: press Space at its foot and hold W.
In our run on Pathbreaker, the fight in the pit lasted about 40 seconds. The key’s item card appeared as soon as the Flame-Crazed Bandit and the Hound Trainer were down, with two Frenzied Hounds still to deal with.
The card lists it as a Story Item named “CAVE PRISON KEY”:
A tarnished brass key, with old mine dust remaining in its grooves.
Long ago, disciples of the Mandate built these cells to confine their own who had undergone Transcension. Now, the cave has fallen into the hands of bandits, imprisoning captured righteous path fighters in these cells.
What the Cave Prison Key opens
Use the ladder to return to the upper floor. The locked cell is roughly 10 m from the ladder’s top. Approach until the E prompt appears, then press E to unlock the door with the key.
Inside the cell are the bodies of prisoners and a chest.
Press E at the chest to open it. It holds a Sha-Chi Amber Shard, whose card reads “Collect all four fragments to increase maximum Sha-Chi by 10%.”
In our run it was the fourth shard, so a second card, “SHA-CHI AMBER”, followed: “Permanently increases maximum Sha-Chi by 10%.”, and the message “Maximum Sha-Chi increased!” appeared.
The upper-floor boards stop beyond the cell at the opening above the pit. Keep walking past the cell and you’ll fall back down into the pit.
The other keys and cranks of the region, with the nearest bell for each, are in the Crimson Forest collectibles guide and the full keys list.
