TBN (exorcist world)
an alternate, sci-fi earth where ghosts and demons are well known to live on the earth. Exorcists are commonplace to disperse resentful ghosts and to seal away demons with the aid of spiritual beasts. However, ordinary humans cannot see ghosts or demons. Most exorcists go through an eye surgery to implant a lens that allows them to see these creatures, but others will wear glasses or contacts with a special lens to aid in seeing them.
Demons are immortal creatures, bringers of chaos and destruction, they are formed from negative energies and resentment, they can be sealed but when they are dispersed they will eventually reform again. Demons can feed on resentment and negative emotions as well as demonic energy, some small few can even feed on spiritual energy
Spiritual beasts are creatures who are sent down or summoned from the heavens/a higher realm. They can be contracted by humans to aid in the dispersing and containment of ghosts and demons. Uncontracted spirit beasts are very unpredictable. Because they come from the heavens, staying on earth without a contract for long periods of time can cause them to become very aggressive and feral, they behave much like wild animals and can cause serious destruction when left on their own. The contract makes them take on a lesser form, and then they draw spiritual energy from their human, the exorcist they are contracted to.
Ghosts are the souls of humans who held on to life whether due to resentment, regrets, or missed hopes/dreams. Ghosts usually disperse after 3 days of being dead, those who do not have a very strong attachment to the world, this attachment is usually negative and these ghosts grow stronger from negative energies, such as those coming from demons, ghosts who linger on in the world often lose their sense of self and behave more like demons, ghosts that are dispersed do not reform. Ghosts can feed on demonic and resentful/negative energy but are usually dispersed by spiritual energy and its purifying ability.
In very rare cases, ghosts have been able to possess a demon which results in a ghostly beast known as a soul swallower. Soul swallowers have lost all sense of humanity but because of their former humanity are able to consume both spiritual and demonic energies, neither will disperse them, which is part of why they are so dangerous. Soul swallowers are a ghostly beast which devours ghostly energy, ghosts and sometimes spiritual and demonic beasts to gain power. They are one of the only predators of spiritual beasts and resemble one very closely in order to not be detected.
Humans are actually able to use either spiritual or demonic energy to become stronger but demonic energy has many negative side effects on a person and is viewed in a less pleasant light