You Dare!*

There were only two reasons why a demon's presence would vanish completely, as far as Kalliēl was concerned.

First, they died – or purified, to be more precise, since demons would not die for no apparent reason. This was the ideal outcome Kalliēl had fought for before he was forcibly summoned to Heaven. He wanted to eradicate the demon all the way down to its core, cutting off its path of retreat, so that it could never return to haunt Rui Ye and the villagers again. 

Second, assimilation.

This case was commonly found in Hell where dark creatures thrived. Unlike the clear-cut hierarchical structure of angels in Heaven, Hell was a lawless and chaotic realm where power ruled supreme. Demons often challenged each other in a deadly battle where the loser would be devoured by the winner. Such was a surefire shortcut to boost one's strength and prestige. 

A prime example was Lucifer, the fallen angel who was banished to Hell after his failed coup.