"Demon Hunter ranks can roughly be divided into Demon Hunter Apprentice, official Demon Hunter, and Demon Hunter Master... To become an official Demon Hunter, one must pass the Black Iron Trial. To become a Demon Hunter Master, one must pass the 'Red Mountain Trial'!"
How did Aaron know?
He found out after getting a Frost Wolf Sect Demon Hunter drunk, who admitted it himself.
"However... I have a vague feeling that there's a problem with the Demon Hunters' Magic Potion configurations. If improved, it might even support another mutation!"
The abilities of a Demon Hunter come from mutation!
If the first mutation is considered relatively simple, then the second mutation requires inducing another transformation on top of the already mutated genes.
Since there's little room for modification, becoming a Demon Hunter Master is very difficult.
And to break through above Master level, one must induce a third mutation on the basis of the second mutation!