It is a noun.
Meaning: witchcraft or black magic i.e. a method of communicating with the dead or black art. It is in general magic that is practiced by witches or sorcerers.
necromancer - noun.
necromantic, necromantical - adjective.
Synonyms: sorcery, (black) magic, witchcraft, wizardry, occultism, enchantment, divination, demonology,hocus-pocus,jinx,spell
It is surprising that people still believe in weird practices like necromancy even in this twenty first century.