Just like Horizon, The Netherworld has several creatures as well that have unique traits and powers.
Some sensitives have been found to turn on the legion and join sides with the demons. These humans are called Seirs. This group cannot be harmed by any angels because the angels have sworn to protect ALL mortals. Seirs will find angels that are out on missions and kill them while the guardian is defenseless. Should a guardian kill a Seir their soul will die the True Death.
Also called deviling, fiends are powerful and evil supernatural being, not to be mistaken for demons. Fiends are highly intelligent and more powerful than even the higher demons. There are a lot more demons then fiends and there is a chance an Angel will never come across a fiend ever. That being said, there are still several types of fiends.
These Fiends can transform into living beings. They are found in the Black Wastes on the Netherworld.
Archfiends are the equivalent of the Archangels of horizon. They are the commanders of the Demon Legion. No known archfiends have been killed by Angel hands.
Morthdu is the mother of all darkness. It is the keeper and heart of the Netherworld. Morthdu is a green sun-like entity with a consciousness said to have excited since the beginning of time.
Also known as Death Demons, Reapers are evil creatures form the lower plane of the Netherworld. They wield a scythe that is known to cut the ties from an angel or mortal and their souls causing the true death before the soul is devoured.