Necromancy or nigromancy is a form of magic involving communication with the deceased, either by summoning their spirit as an apparition or raising them bodily for the purpose of divination, imparting the means to foretell future events or discover hidden knowledge, or to use the deceased as a weapon, as the term may sometimes be used in a more general sense to refer to black magic or witchcraft.

The word "necromancy" is adapted from Late Latin necromantia, itself borrowed from post-Classical Greek (nekromanteía), a compound of Ancient Greek nekros, "dead body", and "prophecy or divination"; this compound form was first used by Origin of Alexandria in the 3rd century CE. The Classical Greek term was nekyia, from the episode of the Odyssey in which Odysseus visits the realm of the dead, in Hellenistic Greek, rendered as necyomantīa in Latin, and as necyomancy in 17th-century English.

Early necromancy was related to and most likely evolved from shamanism, which calls upon spirits such as the ghosts of ancestors. Classical necromancers addressed the dead in "a mixture of high-pitch squeaking and low droning", comparable to the trance-state mutterings of shamans.

Necromancy was prevalent throughout Western antiquity with records of its practice in Babylon, Egypt, Greece, and Rome. In his Geographica, Strabo refers to necyomanteis, or "diviners by the dead", as the foremost practitioners of divination among the people of Persia, and it is believed to have also been widespread among the peoples of Chaldea (particularly the Sabians, or "star-worshipers"), Etruria, and Babylonia. The Babylonian necromancers were called manzazuu or sha'etemmu, and the spirits they raised were called etemmu.

The oldest literary account of necromancy is found in Homer’s Odyssey. Under the direction of Circe, a powerful sorceress, Odysseus travels to the underworld in order to gain insight about his impending voyage home by raising the spirits of the dead through the use of spells which Circe has taught him. He wishes to invoke and question the shade of Tiresias in particular; however, he is unable to summon the seer's spirit without the assistance of others. The Odyssey's passages contain many descriptive references to necromantic rituals: rites must be performed around a pit with fire during nocturnal hours, and Odysseus has to follow a specific recipe, which includes the blood of sacrificial animals, to concoct a libation for the ghosts to drink while he recites prayers to both the ghosts and gods of the underworld.

Practices such as these, varying from the mundane to the grotesque, were commonly associated with necromancy. Rituals could be quite elaborate, involving magic circles,wands, talismans, and incantations. The necromancer might also surround himself with morbid aspects of death, which often included wearing the deceased's clothing and consuming foods that symbolized lifelessness and decay such as unleavened black bread and unfermented grape juice. Some necromancers even went so far as to take part in the mutilation and consumption of corpses. These ceremonies could carry on for hours, days, or even weeks, leading up to the eventual summoning of spirits. Frequently they were performed in places of interment or other melancholy venues that suited specific guidelines of the necromancer. Additionally, necromancers preferred to summon the recently departed based on the premise that their revelations were spoken more clearly. This time frame was usually limited to the twelve months following the death of the physical body; once this period elapsed, necromancers would evoke the deceased’s ghostly spirit instead.

While some cultures considered the knowledge of the dead to be unlimited, ancient Greeks and Romans believed that individual shades knew only certain things. The apparent value of their counsel may have been based on things they knew in life or knowledge they acquired after death. Ovid writes in his Metamorphoses of a marketplace in the underworld where the dead convene to exchange news and gossip.

There are also several references to necromancers called "bone-conjurers" among Jews of the later Hellenistic period in the Bible. The Book of Deuteronomy, explicitly warns the Israelites against engaging in the Canaanite practice of divination from the dead.

Though Mosaic Law prescribed the death penalty to practitioners of necromancy (Leviticus 20:27]), this warning was not always heeded. One of the foremost examples is when King Saul had the Witch of Endor invoke the Spirit of Samuel, a judge and prophet, from Sheol using a ritual conjuring pit (1 Samuel 28:3–25]). However, the so-called witch was shocked at the presence of the real spirit of Though Mosaic Law prescribed the death penalty to practitioners of necromancy (Leviticus 20:27), this warning was not always heeded. One of the foremost examples is when King Saul had the Witch of Endor invoke the Spirit of Samuel, a judge and prophet, from Sheol using a ritual conjuring pit (1 Samuel 28:3–25). However, the so-called witch was shocked at the presence of the real spirit of Samuel for in I Sam 28:12 it says, "when the woman saw Samuel, she cried out in a loud voice." Samuel questioned his reawakening asking, "Why hast thou disquieted me?"  Saul did not receive a death penalty but, he did receive it from God himself as prophesied by Samuel during that conjuration.

Some Christian writers later rejected the idea that humans could bring back the spirits of the dead and interpreted such shades as disguised demons instead, conflating necromancy with demon summoning. Caesarius of Arles entreats his audience to put no stock in any demons or gods other than the Christian God, even if the working of spells appears to provide benefit. He states that demons only act with divine permission and are permitted by God to test Christian people. Caesarius does not condemn man here; he only states that the art of necromancy exists, although it is prohibited by the Bible. On the other hand, some Christians believe that necromancy is real but, God has not suffered Christians to deal with those spirits (Deuteronomy 18:14). Since those spirits are not operating in the Holiness and pure Righteousness of God, they can be malevolent, hit or miss prophecies that may have some truth to them but in the end, they are complete lies.

Norse mythology also contains examples of necromancy, such as the scene in the Prophecy of the Völva in which Odin summons a völva, or shamanic seeress, from the dead to tell him of the future. In The Spell of Gróa  the first part of The Lay of Svipdagr (Svipdagsmál), the hero Svipdag summons his dead mother, Gróa, to cast spells for him. In the Saga of King Hrolf kraki (Hrólfs saga kraka), the half-elven princess Skuld was very skilled in witchcraft to the point that she was almost invincible in battle: when her warriors fell, she made them rise again to continue fighting. In Hávamál Odin tells of a runic charm that allows him to resurrect the dead. Snorri Sturlasson writes that Draugadróttinn (lord of the draugr) is a name of Odin. A further connection between Odin, necromancy and the undead was the belief that the dead where said to return from their graves during Yule. "Jólfaðr" (Yule father) and "Jólnir" (the Yule one) are also names of Odin. It would make sense in Odin's "necromantic" context for him to be associated with a season of the returning dead.

Many medieval writers believed resurrection was impossible without the assistance of the Christian God. They translated the practice of divination as conjuring demons who took the appearance of spirits. The practice became known explicitly as demonic magic and was condemned by the Catholic Church. Though the practitioners of necromancy were linked by many common threads, there is no evidence that these necromancers were ever organized as a group.

Medieval necromancy is believed to be a synthesis of astral magic derived from Arabic influences and exorcism derived from Christian and Jewish teachings. Arabic influences are evident in rituals that involve moon phases, sun placement, day and time. Fumigation and the act of burying images are also found in both astral magic and necromancy. Christian and Jewish influences are found in the symbols and conjuration formulas used in summoning rituals.

Practitioners were often members of the Christian clergy, though some nonclerical practitioners are recorded. In some instances, mere apprentices or those ordained to lower orders dabbled in the practice. They were connected by a belief in the manipulation of spiritual beings especially demons and magical practices. These practitioners were almost always literate and well educated. Most possessed basic knowledge of exorcism and had access to texts of astrology and demonology. Clerical training was informal and admission to universities was rare. Most were trained under apprenticeships and were expected to have a basic knowledge of Latin, ritual and doctrine. This education was not always linked to spiritual guidance and seminaries were almost nonexistent. This absence allowed some aspiring clerics to combine Christian rites with occult practices despite its condemnation in Christian doctrine.

Medieval practitioners believed they could accomplish three things with necromancy: will manipulation, illusions, and knowledge. Will manipulation affects the mind and will of another person, animal, or spirit. Demons are summoned to cause various afflictions on others, "to drive them mad, to inflame them to love or hatred, to gain their favor, or to constrain them to do or not do some deed."Illusions involve reanimation of the dead or conjuring food, entertainment, or a mode of transportation. Knowledge is discovered through demons, who provide information about various things which include identifying criminals, finding items, or revealing future events.

The act of performing medieval necromancy usually involved magic circles, conjurations, and sacrifices such as those shown in the Munich Manual of Demonic Magic. Circles were usually traced on the ground, though cloth and parchment were sometimes implemented. Various objects, shapes, symbols, and letters may be drawn or placed within that represent a mixture of Christian and occult ideas. Circles were believed to empower and protect what was contained within, including protecting the necromancer from the conjured demons. Conjuration is the method of communicating with the demons to enter the physical world. It usually employs the power of special words and stances to call out the demons and often incorporated the use of Christian prayers or biblical verses. These conjurations may be repeated in succession or repeated to different directions until the summoning is complete. Sacrifice was the payment for summoning; though it may involve the flesh of a human being or animal, it could sometimes be as simple as offering a certain object. Instructions for obtaining these items were usually specific. The time, location, and method of gathering items for sacrifice could also play an important role in the ritual.

The rare confessions of those accused of necromancy suggest that there was a range of spell casting and the related magical experimentation. It is difficult to determine if these details were due to their practices, as opposed to the whims of their interrogators. John of Salisbury is one of the first examples related by Richard Kieckhefer, but as a Parisian ecclesiastical court record of 1323 shows, a "group who were plotting to invoke the demon Berich from inside a circle made from strips of cat skin," were obviously participating in the church’s definition of "necromancy".

Herbert Stanley Redgrove claims that necromancy was one of three chief branches of medieval ceremonial magic, the others being black magic and white magic. This does not correspond to contemporary classifications, which often mistake "nigromancy" ("black-knowledge") with "necromancy" ("death-knowledge").