They still have not discovered that the slimes are the ultimate spies in this world, they are considered as beings that are purely focused on cleaning the filth of this world, there are multiple of them and when they die, there would be magic cores left behind.
The cores would either decompose and turn into pure energy that would submerged into the ground, trying to reach the plane's core as much as possible or would be obtained by some individuals or monsters.
Different cores has different values and since it will only take three days for them to crack and turns into powder without any special preservation skill, it is hard to find them in the wild.
This continuous cycle repeats when slimes cannot evolve into their next cycle and since not many would attack the slimes, they are also ordered not to attack anyone without prior reason.
