Princess Aelin:

About demons: Demons are beings that came from a plane called Yami no Sekai and are hostile to any lifeform not made up of Dark Magic. They come in two verities:

1. Lesser Demons: These demons, while certainly strong and capable of magic, can be permanently killed with normal weaponry.
2. Demon Nobles: These demons are extremely deadly and if killed with normal weaponry they respawn after one solar day (24 hours). If killed with magic or an enchanted weapon, they die for good. The strength is denoted by their titles with a knight being the weakest, and archduke (of which there are only four) being the strongest. Their complete ranks are Knight, Baron, Viscount, Count, Margrave, Duke, Archduke.