Demonology W-X-Y-Z

Warlocks
Water Demon
The Wendigo
The Wicked Witch

The Woogyman
Xavier & Tull
Zahn
Zile

Warlocks
All witches must follow the important rule from the Wiccan Rede: 'An harm it none, do as ye will'. When a witch crosses this line, and uses his magic to harm others, he or she will cross over to the dark side and become a warlock. A warlock's only goal is to kill good witches, and obtain their powers.
Long ago there was a witch who had the power to teleport herself within the blink of an eye. A warlock killed her and stole her power; now this power, called 'blinking', has become more and more associated with warlocks and even a power you can recognize a warlock on.
A child born to two warlock parents will automatically be a warlock himself. The only way to lose this warlock nature, is to embrace God and become a priest.
 
The Water Demon
A demon who dwells in water, and is made of water itself. It kills his victims by drowning them from the inside. The water demon has the ability to posses his victims, an ability he sometimes uses to lure his victims to the water by taking on the form of an innocent person, pretending to be drowning.
This evil is difficult to fight due to its immunity for witches' powers. The only way to kill the demon is to use electricity to separate the water particles that make up the demon.
The Wendigo
The Wendigo looks like a normal person during the day, but at night, it transforms into a vicious creature, sort of like a werewolve, only much meaner. During the three phases of the full moon, the Wendigo haunts down people with the bloodgroup AB negative, who it kills before eating their hearts in order to survive.
The first Wendigo was a mortal who betrayed his lover, cut out her heart and ate it. As soon as he did, his own heart turned to ice and that's how he became this monster.
The only thing that fights of this demon is bright light, like fire. When one is scratched by a Wendigo, the person will turn into one himself within hours. To unbecome a Wendigo (and kill the original Wendigo himself), melt the original Wendigo's heart of ice.
The Wicked Witch
The Wicked Witch of the Enchanted Forest. She used her magic to curse magical beings into all sorts of weird forms, so they couldn't perform their magical duties.
The Woogyman
The Woogyman is a shadow, and it dwells in the basement of the Halliwell Manor. It has the power to posses people, granting them with great powers as well. The Woogyman's goal is to take over the Halliwell Manor. Since the house is located inside a pentagram formed by the natural elements, it's not only a spiritual nexus but a wiccan one as well, meaning it is a battleground fro good and evil. When evil gets oppurtinity to take over the mansion, it becomes more powerful and spreads.
The Woogyman can be freed during an earthquake, when his prison beneath the floor under the manor is destroyed.
Xavier and Tull
These two demon brothers, seeking revenge for their father's death, targeted a group of wood nymphs, trying to gain access to the Eternal Spring, so they could drink from the spring and become invincible.
Zahn
A lower level demon and a soul trader. He specializes in demonic contracts and demonic law.
The past few years Zahn became quite powerful because of the Faustian contracts he made with mortals. The contracts guaranteed that the mortal would become wealthy in life, in exchange for his or her soul after death. Once he receives a new soul, zahn sells them to demons at an auction, in exchange for various magical powers, making him more and more powerful.
Zile
A warlock with the power of shapeshifting. He teamd up with a dark priestess called Dantalion, to trick Prue into marrying him in a dark ceremony causing Prue to turn evil. This would automatically turn Piper and Phoebe evil as well, including the Halliwell Book of Shadows, allowing Zile to grant the ancient Halliwell magic.