Devil's Playground (デビルズ・プレイグランド Debiruzu Pureigurando?) are a series of laboratories that conduct experiments and perform modifications on children. In these laboratories, children would be tasked to fight and terminate the other children partaking in the game in order to determine the strongest test subject. The winning test subjects would then be sold to the highest bidding person, whether it be to serve in a pirate crew, be a slave to a kingdom, or a member of a noble family. Many of the Playgrounds were destroyed by the Revolutionaries, leaving only six in total. Each test subject is code-named after a fairy tale or nursery rhyme.
Orphans or children who are no longer in the company of adults are usually found by Takers. From that point, they are brought to a Playground and branded with a code number and given a code name to help identify them. They are assigned a room with two other children and are allowed to interact with one another depending on the rules of the Master. To begin experimentation, children are injected with a unique drug called Cyanine (イコール Shianin?) (due to the fact that it turns blood cyan) which increases their life force and durability to allow for dangerous procedures. It also functions as an enhancer, enhancing things like speed, strength, and reflexes. From then on, children are forced to fight and participate in obstacle courses to demonstrate their new modifications. Many affiliates donate their money for certain experiments and modifications to be done for a child, usually purchasing them if it ends in success. Those who cannot handle the weight of their modifications are deemed as failures and are usually terminated or locked away forever.
Children who have been locked away are put into the underground vaults, growing into adults that never experience the outside world.
- Playground 01: Located on an unknown island in the Grand Line, this playground is known for it's large size and successful experiments. The master of this playground does not practice the "King of the Hill" game, which involves the test subjects murdering each other.
- Playground 02: Located on a winter island in the Grand Line, this playground is considered calm and is ruled by a master who believes in fairness.
- Playground 03: Located on a summer island in the Grand Line, this playground is considered defective, as the test subjects have murdered their master and taken the playground for themselves.
- Playground 04: Located on an unknown island in the Grand Line.
- Playground 05: Located on an unknown island in the Grand Line.
- Playground 06: Located on an autumn island in the Grand Line, this playground is considered the worst one of all. Many test subjects do not make it due to the severity of the torture they face from their master. Those who do escape from this hellish playground are left with deep psychological scars and fracturing.
|"Rabbit"||"Beast"||"Nimble Jack"||"Snow White"||"Gretel"|
|"Hansel"||"Jack"||"Hardgore Alice"||"Aphrodite"||"Little Boy Blue"|
|"Biscuit Mistress"||"The Masked Man"||"Unseen Princeps"||Caribal Kingdom|
- Inspired by Six.