c. 1300, nygromauncy, nigromauncie, "sorcery, witchcraft, black magic," properly "divination by communication with the dead," from Old French nigromancie "magic, necromancy, witchcraft, sorcery," from Medieval Latin nigromantia (13c.), from Latin necromantia "divination from an exhumed corpse," from Greek nekromanteia, from nekros "dead body" (from PIE root *nek- (1) "death") + manteia "divination, oracle," from manteuesthai "to prophesy," from mantis "one who divines, a seer, prophet; one touched by divine madness," from mainesthai "be inspired," which is related to menos "passion, spirit" (see mania). The spelling was influenced in Medieval Latin by niger "black," on notion of "black arts;" the modern English spelling is a mid-16c. correction. Related: Necromantic.
late 14c., nygromanser, nigromauncere, "sorcerer, adept in black magic," from Old French nigromansere, from nigromancie (see necromancy). Properly "one who communicates with the dead" but typically used in a broader sense in English.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nasyati "disappears, perishes," Avestan nasyeiti "disappears," nasu- "corpse," Old Persian vi-nathayatiy "he injures;" Greek nekros "corpse;" Latin nex, genitive necis "violent death, murder" (as opposed to mors), nocere "to harm, hurt," noxius "harmful;" Greek nekus "dead" (adj.), nekros "dead body, corpse;" Old Irish ec, Breton ankou, Welsh angeu "death."
1590s, "mathematical figure (presumably originally one of five points) used in magical ceremonies and considered a defense against demons," from Medieval Latin pentaculum "pentagram," a hybrid coined from Greek pente "five" (from PIE root *penkwe- "five") + Latin -culum, diminutive (or instrumental) suffix. OED notes other similar words: Italian had pentacolo "anything with five points," and French pentacle (16c.) was the name of something used in necromancy, perhaps a five-branched candlestick; French had pentacol "amulet worn around the neck" (14c.), from pend- "to hang" + a "to" + col "neck." The same figure as a pentagram, except in magical usage, where it has been extended to other symbols of power, including a six-point star. Related: Pentacular.