Lucid is the substance that makes up the sphere known as Lucid Adventure.
When the Gods were killed 14 years ago, the boundary between the real world and Lucid Adventure fell apart. This led to an inflow of Lucid from the Real World that increased the size of Lucid Adventure by 10 times.
The increased land mass is being fought over by the various powers in Lucid Adventure.
Notes & Trivia
- Line Webtoon was not clear were Lucid came from but in the Korean version, Heart Heater says "from your world" when talking to Sora, indicating it's from the real world.