Monster Invasion

Twenty years ago the world experienced a great tragedy. 
A strange place called Devil Domain suddenly appeared. It brought with it countless Devil Caves around the world which spawned monsters that caused havoc everywhere. 
These monsters were immune to human weapons so they could only be defeated through unconventional means. 
That is how the Devil Domain Expert came to be. 
These were individuals who fought and gained strength in the Devil Domain. 
They would also have special abilities which they could retain even when they came back to the real world.
Zhang Ze came to this world ten years ago and became the Zhang family’s adopted son. 
His adoptive parents died five years ago during a Devil Cave incident and left him alone with his little sister. 
She was not in good health and Zhang Ze needed a lot of money to take care of her.
He was eighteen years old this year and placed his hopes on becoming a Devil Domain Expert with a high-level innate skill. 
This will earn him enough money to get his sister the medical help she so desperately needs.