Very few of the double mutated monsters would possess a forbidden skill. Such a skill was different from the rest as it would normally have rather harsh conditions of using it and many of them even left side effects on the users.
The Fire Phoenix's fire skill could only be used once after having accumulated flame energy for more than a hundred years. Despite the fact that it had immense power, it was somehow restricted from using it simply by environmental factors.
This was because the forbidden skill relied on the absorption of external heat energy to generate its power. It would become more powerful in places with an adequate supply of heat energy. On the contrary, in regions with extremely low temperatures, the attack would do nothing to its opponent as the effect would be even worse than an ordinary skill.
