Demons and ghosts are difficult things to write.Possible complications could be caused if you base it off of a real life haunting.Both of these are immensely different,so I will develop two groups below.
Demons:
Demons in real life are an incredibly difficult subject to write about.In fiction,they are generally malicious and attempt to harm characters.
A go-to way of demon creating is making one of your own.I recommend this because,if you write about a real demon(such as Zozo)you may offend genuine believers.
The major difficulty of a demon in your story is you may end up with a flat character.Why are they tormenting the protagonist(s)? What about the characters draws them in?How do they work?
In general,you need creativity,motivation and a major lure for your demon character to attack the character(s).
Ghosts:
Spirits,ghosts and other types of supernatural entities can be from any class,age,religion and time period.
The classic ghost story is a haunted house with tormented souls encapsulated inside.It is up to our family(sometimes a single young man)to defeat or help them.
Of course,you could switch up details.For example,have an elderly woman face off against the horrifying ghostly mother of a family.The mother could have killed the rest of the family and is trying to repeat it with the woman's daughter.
An extra lure of writing ghosts is that they can do anything the writer needs.They can be visible/invisible,able to use physical objects/can't,the possibilities are endless.
In general,writing any spirit is difficult.Of course you can have other spirits and supernatural entities;I just wouldn't recommend writing about bek or wendigos on your first time.
