Unique beasts

"What is a life and death butterfly?" Zhang Zi asked.

"Not a life and death butterfly. The life and death butterfly." Bai Yun clarified. "It is a unique beast."

"What is a unique beast?" Chen Jin asked.

"Most beasts in the universe are members of some race. Which means that there are many members of those beasts. Even if a never before seen variant appears, it doesn't mean that such a variant cannot appear later." Ao Tianzi explained.

"But for unique beasts, only one such can exist at a time. Only when the beast dies can another be born. That is because each of these beasts has dominion over one part of existence. That means that these beasts are guaranteed to reach the god step."

"For example, the hydra, which is a unique beast, has dominion over the concept of multiple elements. No beast can use multiple elements as well as the hydra. Then there is the phoenix which has dominion over fire."