No such thing
As Ghosts. As far as we think about dead people walking (or floating) around the earth. No one who has died is allowed to return or has come back. At least with the possible exception of Samuel at En Dor, to give a message of death to King Saul.
Now evil spirits do exist, but these are what the bible call fallen angels. These spirits have been around for thousands of years since the fall, and they know quite a bit about influencing mankind.
Mediums cannot speak to any departed soul. Most are charlatans and the few who think they can are actually doing so with evil spirits.
The threat lies with the living, but don’t play around with the darkness.