Sneakily, Phantom Blade Zero is a Metroidvania game. You’ll regularly need to backtrack to places you’ve visited before to use newly acquired weapons or open doors you’ve found keys for. The cell key in Phantom Blade Zero is the first item you’ll be looking for, and acts as an introduction to exploration in the game. Thankfully, the key you’re looking for isn’t too far away from the cell door itself.
Cell key location in Phantom Blade Zero
After you’ve found the door, you need to progress through the Weeping Creek Valley. You’ll speak to the Order member and be let through the gate into the outpost. Follow the path around, meeting the wise archer character and learning how to upgrade your weapons for the first time.
You’ll soon stumble across the Outpost bell, which is on a path where the road is blocked to the left by two guards, and another path to the right is blocked by chains. From here, head up the stairs going north, through an archway.
From there, head up the path to the right, and then up another set of stairs to a wooden house. You’ll see some more stairs on the right side of the house, so head up those.
Make your way to the top of the building, and you’ll see a glowing orange orb on the ground. Interact with it to pick it up, and you’ll have the Cell Key added to your inventory.
Now, all you need to do is head back to the entrance of the outpost and use the key to open the cell door. Head inside and you’ll see a purple orb on the ground. This will be your first Blackstone Fragment. You’ll find them throughout the game, and whenever you collect four of them, your attack and guard-break will be increased by 10%.
This is the first time you need to backtrack in the game. I talked about the impact of that side of the game in my long Phantom Blade Zero preview, and it only gets more impactful as the game progresses.