In his talk with the Demon King Lucius, Damien had learned a great deal about the Demons' current situation.
Basically, these mortal Demons were sheep to be herded and separated based on how much they could provide.
Although these Demons were born with the Nox corruption within them, not every Demon had the qualifications to receive the power of the Demon God.
The way it worked was simple. Every 10 years, the Demons above the age of 16 and below the age of 26 were to gather together for a sort of right of passage.
During that right, the present demons would be bathed in the Demon God's power. It was from this point where things changed.
For some, that power would simply flow through them and disappear back into the earth. These demons would then have their memories wiped before they were sent back to Acier to continue their normal lives.