Demonology is the arts of summoning and studying Demons.

Demons are, indeed, usually terrifying creatures. They are either risen back to life by necromancy or empowered by the Dark Arts, also known as Black Magic. Some of the more infamous demons are the Lethifolds and Inferi.

The Arts of Summoning a Demon requires a special Summoning material - usually a stone or cloth.

Therefore, Merfolk are referred to as Demons, because a special parchment, a mirror, and an incantation can Summon them. 

Most Demons require incantations to Summon with a Summoning material. The Succubus is a Demon similar to Fallen Angels and Harpies, and attracts other creatures with its beauty. When a Demon is Summoned, it serves the Summoner, and are then called Daemons. 

Daemon perish when attacked by a Holy creature or weapon, but Demons are more powerful, as it is wild and does not have a master to serve - they require advanced Holy or Divine enchantments to weaken, making it vulnerable to any type of Holy attack.

When a Daemon perishes, it leaves a part of itself behind. These are called Sealstones. A Sealstone is usually found when a spirit or ghost is Sealed into a stone by Witchcraft, but Sealstones are also left behind by perished Daemon. Therefore, a Daemon can return when the five other Sealstones are consumed for an enchantment to return a Daemon from its Sealstone.

Demons leave nothing but black ash behind when it perishes, but it doesn't disappear completely from the world. It returns to the Underworld and stays for a certain amount of time, depending on amount and severity of its sins.