Throughout the ages, in nearly all cultures of the world, humanity has placed the blame for all that is wrong on malicious spirits of the Otherworld. Demons bring plagues and natural disasters, sow discord and encourage hatred. When a deed of particular evil is done, it was inspired by the Devil, not Man. Demons are unhuman, possessors of superhuman powers and inhumane feelings. They are focal points of Evil; their only goal is pain and destruction.
Under the threat of this diabolical force, mankind has attempted to ward off or expel, to master or placate. Man is clever, he knows the power of knowledge. And so he has sought to learn and understand so as to stand a better chance against demonkind.
...Sytry
Satan
Pytho
Pazuzu
Merihim
Mammon
Lucifer
Lilith
Bune
Beliel
Belphegor
Behemoth
Beelzebub
Barghest
Astaroth
Asmodeus
Alü
Agares
Abbadon...
To most people, a "demon" equals with an evil spirit. This is not enough for serious demon research. The belief in demons comes from animism; however demons have acquired more personality in many places since then. The Mesopotamian peoples saw the world as swarming with demons of odd and imaginative shapes. In Judaism, the various gods that other peoples worshipped were called demons because they were not true gods. In Catholic theology, a demon is the opposite of an angel, so a spirit who fell with Lucifer.
Since knowing a being's true name also gives power over that being, much studying
went into finding out the demons. They were listed in grimoires and placed in demon hierarchies, one more intricate than the other. Johann Weyer did one hell of a job when he made an estimate of 7,405,926 existing demons.
Protection from demons
Little magical artifacts are often thought to ward off demons. Amulets are the most used in all cultures, often associated with some form of spell. People in Babylonia owned demon bowls inscribed with incantations agains the demons. Similar demon catchers could be used to curse an enemy.
Some kinds of herbs were effective demon repellants. In British herb lore, angelica, bettony and peony wood were charms against evil in general, while nettle, onion and St. John's wort protected specifically against demons. Several other symbols and materials were used to the same effect. Iron gave power over the demons, as did the pentagram.
More drastic measures against persistent demons included offering them a sacrifice. If the demon received a cock, a goat or a small child, perhaps it would leave the others alone.
And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour. (Matthew 17:18)
Unfortunate people have become possessed by demons from ancient times until today. An exorcist would need great skill, wits or spiritual powers in order to drive out the invading spirit. A preventive method to cleanse places of demons before they went into peole may seem better. The Japanese Tsuina or Oni-yari ceremony aims to scare demons out of the emperor's palace.
Summoning a demon
This is perhaps the real demonology, and sounds like an incredibly arrogant thing to do, considering the power of demons and the means most people use to stay away from them. Still, recipes for calling demons can be found in several legendary works such as the Black Book or the Necronomicon.
According to popular imagination, it is the hobby of every self-respecting occultist to summon a demon in his own basement. The reason behind this is to prove one's elite demon summoning skills or to request a favour from it. The result is generally horrible.