Genesis- Beginning of Earth

This world is older than any of you know. Contrary to popular mythology, it did not begin as a paradise. For untold eons demons walked the Earth. They made it their home, their... their Hell. But in time they lost their purchase on this reality. The way was made for mortal animals, for, for man. All that remains of the old ones are vestiges, certain magicks, certain creatures... - Giles

 Contrary to popular belief, the Earth was not created as a paradise. It was inhabited by pure demons and darkness, The Old Ones. They roamed the Earth bringing destruction and ruled above it all in glory. For eons, the demons walked the Earth and made it their Hell. However over time the Earth made its way for the next dominant species- mortal humans and mammals. The darkness and pure demons were pushed back and diminished in their forces. Demons left to other dimensions while others stayed, hiding in wake for when Old Ones would return and reclaim the Earth as Hell. Demons that stayed adapted, became hybrids or lived in the darkness hiding from the light. Some took on the form of humans, absorbed into their society and thrived. Only traces of the Old Ones remain in the forms of demon hybrids, dark magic and energy.

Vampires

The books tell the last demon to leave this reality fed off a human, mixed their blood. He was a human form possessed, infected by the demon's soul. He bit another, and another, and so they walk the Earth, feeding... Killing some, mixing their blood with others to make more of their kind. Waiting for the animals to die out, and the old ones to return. - Giles

Eons ago, when the Old Ones began to diminish and mortal man emerged, a pure demon fed on a human, mixing their blood, thus rose the first vampire on Earth. In result the vampire bit another and another turning them into vampires as well. They spread rapidly feeding and turning more until their numbers were vast. Impure and hybrid demons, they fed on the blood of the living. Usually killing to feed they also turned humans into vampires by feeding on their blood but not all of it then feeding the human their blood. When they rose again they would be vampires, demons inhabiting the body and personality of the deceased but without a soul, conscience or remorse. Without a soul, these demons preyed on humanity, roaming the Earth killing not only of hunger but for pleasure. 

A vampire isn't a person at all. It may have the movements,  the memories, even the personality of the person that it took over, but it's still a demon at the core, there is no halfway.- Giles (Angel)

As vampires they were immortal, never aging and could only be killed by direct sunlight, fire, decapitation or a wooden stake in their heart. Once killed they would burst into ashes and dust leaving no body. Although immortal these demons still feared crosses, holy water and the Slayer, a hunter with the strength to destroy them. Hiding from day, they lived underground until the sun set when they would escape into the night, ready to feed. As a vampire, they have remarkable speed, strength and healing powers beyond humans which could only be matched by the Slayer.

Buffy: What else what?
Xander: For vampire slayage.
Buffy: Oh, fire, beheading, sunlight, holy water, the usual.

Xander: Okay, so, crosses, garlic, stake through the heart.
Buffy: That'll get it done. - (The Harvest)

While vampires usually have little purpose driven only desire for blood, some such as The Master have had the power, leadership and skill to gather numbers of vampires to one purpose. Vampires rarely gather in large numbers preferring to hunt alone or in small groups. Few vampires have a rare power of hypnosis on their victims, putting them into a trance before feeding on them.

To make you a vampire, they have to suck your blood. Then you have to suck their blood. It's a whole big sucking thing. Mostly, their just gonna kill you. - Buffy

Demons

At the beginning of time, the Earth was ruled by pure demons, the Old Ones who were larger and more powerful than the hybrid demons of now. As the way paved for Man, the pure demons diminished, residing to other hellish realms while others became, hybrid demons and took on human forms. Like the mortal species, the demons evolved into vast subspecies. They spread over the Earth, taking on  human form to live in the new world or lived underground in hiding. With thousands of demon species on the Earth, they were each different with unique attributes and powers. Among many were vengeance demons, chaos demons and many more varieties, some worshipped greatly by cults, some living as tribes while some became part of human society and earned a living. 

While some demons are concerned only with plaguing humanity, others have found benefits in the new world and become helpful members of society. Some are good such as Lorne with the power to help and guide people. The Powers That Be have chosen some demons to be their link and messenger. Unlike vampires, demons have the choice to be whether good or evil and have a conscience. Many demons have acquired a power to resume a human appearance as a result of adaptation.

Dimensions

 Beyond our world lies uncountable dimensions, each living separate from each other yet all walking the same line. With each action we make countless dimensions are created. There are dimensions where one simply action could lead to a world without shrimp or a world ruled by demons. We are unable to see these other dimensions existing alongside ours not even knowing if they exist but just occasionally we reach out to touch them. 

With a variety of dimensions they include hell-dimensions, heavenly dimensions and more, some with demons and gods while others are ruled by humans. With infinite possibilities it is impossible to grasp one from another.  Each dimension has its darkness and light ruling. Some are host to demons while others home humans or gods. Hell-dimensions are mainly dimensions ruled by evil while heavenly dimensions are ruled by light. 

Each separate dimension runs its course alongside an infinite other dimensions, separated by walls that cannot be seen. Thus we are unable to know or even comprehend that ours may not be the only world. However while the walls to the dimensions can block invaders or visitors, they can be broken down by using spells or rituals that will allow beings to travel through them. If the incorrect spell is used it could lead to the travelers being taken to a completely different destination lost within the fabrics of time and space forever. Certain spells revolve around playing with the dimensions such as teleportation spells though the destination could vary from troll-land or a land without shrimp. There are certain weaknesses in the walls, where the fabric between the dimensions fades, located around the world were if weakened just enough could break through. Some areas have a huge amount of mystical energy which is useful for travelers but once used it must regain the energy.

When a being is teleported or pulled through dimensions it may end in disastrous consequences where they return deformed. When a being travels through the walls they're atoms and molecules are broken down then reassembled afterwards. If the atoms and molecules are not assembled properly they may return deformed or different. In the case of more than one being traveling at a time it could lead to a mixed and freakish whole form. Some beings have the ability to travel through the dimensions of time and space easily while others are able to sense the areas where there are potential breakthroughs.

Heaven and Hell

With the vast numbers of religions and cults emerging, the existence of heaven and hell can only be more controversial. However it is believed that there is heaven and hell. Lots of them. Among the infinite number of dimensions, humans have come to know some as what they imagine as hell or heaven. There are many different hell-dimensions and heavenly dimensions though their is great controversy over the belief of this. Because we perceive hell as being a place of torment and suffering, this could easily be confused with one of many dimensions ruled by demons and evil. On the other side, we perceive heaven as a place of pure joy and love, confusing it with another dimension. 

When a being dies, their soul is taken to basically heaven or hell. It is not known where either of these are or whether they even exist. Their is also the existence of the Ghost Roads, wherein lost souls travel aimlessly until they are ready to let go and enter heaven. Or hell. Some cultures believe in reincarnation, wherein when the body dies the soul is transferred to a new body to begin a new life. Lives are replayed over and over again.

Heaven and hell are also known as other planes. Where a being becomes a higher being for evil or good, they no longer belong to their original plane and are promoted to a higher existence where they can fulfill their full potential. Hardly anything is known about the planes or their existence. Unlike dimensions, planes are not physical and movement between them are near impossible. We like to think that dimensions run side by side whilst planes are on top of each other. In a non-physical plane, it is suspected that their are no physical needs or existence rather a feeling that exists their to do its purpose. Higher beings are able to see things in a new light that mortals find hard to comprehend.