Because this demon's ability was to turn into a real ghost.
So, what is a real ghost?
Very simple.
They could fly around weightlessly.
They could pass through walls as if they had no physical form.
Physical means of the real world couldn't touch them in the slightest.
Moreover, they could become invisible, torment others in their dreams, curse others, cast resentments, and even possess others' bodies—all of it was doable.
Therefore, Milu was known as Ghost Demon and was considered one of the most dangerous among the Six Great Demons, alongside Lide.
Whenever she wished, she could turn an entire huge city into a Ghost Realm, suck out all the people's life force, and even capture their souls, turning them into her Ghost Army and slaughtering all living beings.
This was the Unique Skill of Ghost Demon Milupesh—[Ghost Species].
