Only those who are kings or almost like a king wear black clothes.

The act of proposing marriage among the aristocrats of the Demon Tribe is slapping the other's left cheek.

Dropping or picking up a knife means the person is consent to a duel.  Add in a fork, and pointing the two items at your opponent is to announce that you have taken your opponent's lover.

Demons are born with magic power.  Most of the time, they just specialize in one element like fire, earth, etc.