"Do not turn to mediums or necromancers; do not seek them out, and so make yourselves unclean by them: I am the LORD your God.
Mediums are those who use spirits to gain information. Necromancers are those who communicate with the dead, also for information gathering or for the desires of people to contact 'departed' loved ones. Very often people would want clarification on inheritance claims or to find a lost treasure which the dead person had buried. Others would just want to know their future.
The use of mediums and necromancers is wicked because they attempt to usurp power and glory which God has reserved for himself. God is the only one who is all-knowing and God is the only one who has power over life and death. Also, God is the source of all blessing and using these means to seek wealth is to seek blessing in a wrong way.
It is important to note that the medium or necromancer does not actually have control over the spirits as they suppose. They are merely deceived by demons to think they have power when they don't. Ghosts are deceptive spirits in the biblical framework of understanding the world. Demons like to distract from the truth and to mislead people so that they fail to worship God as he expects them to.