Limbo Stone

Limbo Stones are rare crystals that can only be found in Limbo, a separate space between the Underworld and human world that is impossible for organisms other than Gravekeepers to survive in.

Description
Limbo Stones create their own dimensions by absorbing the energy of the environment and consciousness of the organisms around it, most of the time being Gravekeepers' since they are the only organisms able to survive in Limbo. The rules and environment within the dimension are decided by the creator's conscious and subconscious, so even if the creator doesn't intentionally create hints for the rules in the dimension, hints will still appear in the dimension. Creating the environment in a dimension with a Limbo Stone takes a lot of time and effort, so only people who have a lot of spare time, enjoy world building and can survive in the Limbo can create Limbo Stones with dimensions.

When created, the pocket dimension is still only survivable for Gravekeepers, unless it is crushed. When crushed, the dimension will become survivable for all organisms but will only last three days. While in the dimension, people who don't meet the entrance requirements will experience effects similar to being in Limbo and almost instantly die. After three days, anyone who is still within the dimension will randomly appear at a random location either in the human world, Underworld or Limbo if they don't meet the exit requirements. If they do meet the requirements, they can choose to travel to either the human world, Underworld or Limbo, though the location they teleport to is still random.

Known Limbo Stone Dimension Rules
The Limbo Stone dimension used by Shi Ji: Must be biologically under twenty years old to enter. Creatures that weigh more than ten kilograms and are longer/taller than one meter can not enter forests or deep water. A person must kiss someone or be kissed by someone to exit.