Necromancy is not as bad as it sounds. It is magic involving the dead, usually to divine the future or discover hidden truths. It therefore covers all those practices in which a medium summons the spirit of a deceased person or a priest calls upon a long dead prophet to provide guidance and wisdom.
However some definitions of the word paint it in blacker terms, as witchcraft involving the raising of dead bodies. Professor Van Helsing no doubt considers it rather more serious than getting out the Ouija board.