Nekromanteia: Homer's Odyssey Book 11 is the founding scene, Odysseus summoning shades with blood. Greek nekya rites, Roman and medieval evocations, and the Munich Manual's operations continued the genre. The -mancy suffix, manteia, seer-craft, spawns the whole divination suffix family: pyromancy, geomancy, hydromancy, cartomancy.
The semantic decay: by the Renaissance, necromancy slid from dead-divination to any sinister sorcery, corrupted via nigromancy, black-mancy, a spelling error that rewrote the meaning. Hollywood finished the job; the word now means skeletons on command.
The logic the practice encodes: the dead are the complete record, every witness, every secret, no longer able to refuse. Necromancy is archival research with a ritual login, and its modern heirs, séance, mediumship, the Ouija board, are the same query with friendlier interfaces.
Why does every culture believe the dead know something, and none agree on what?