The World's cancer, a 'biological' creation using Zheng Yichen as a sample, possesses a Boundary Bubble and a deteriorated Life Furnace, becoming a monster capable of surviving and growing even when cast beyond the World.
If this thing is not promptly exterminated and suppressed, it will rapidly proliferate malignantly and destroy a World in no time, just like the Boundary Breaker that Zheng Yichen encountered before.
Placed in the Outer Realm environment, it can also continuously proliferate and grow, eventually becoming a cancerous monster devouring the World. Cancer requires nutrients to sustain itself.
For this thing, the World itself is the best source of nutrients.
Once this thing goes out of control, it is a disaster. Ahn Ke is very satisfied with her creation. Although its appearance is hideous to the extreme, its essence is the World's cancer capable of destroying Worlds.
