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Evolution.
The ability for a living being to surpass their limits and provoke a whole change in both their bodies and souls.
Evolution is fairly common with monsters, in earlier stages and ranks of development, they evolve the most, and as they grow, they begin to evolve less and less.
This is because evolution requires a large quantity of power accumulated until it reaches the limit that the current body can take.
Of course, in early stages of development, monsters are able to evolve constantly because their bodies are still weak and their energy needs to be constantly expended.
When this happens, this amount of energy in the body is used to make the body forcefully adapt to this amount of power, evolving and ultimately making it stronger, additionally, most of this power is also used through the evolution, leaving the monsters that evolve often tired after doing so.