Dank Christian Memes

You are describing osteomancy.

Divination via some kind of manipulation of bones.

Necromancy is much more broad, and encompasses any kind of ritual or practice that is performed with the aim of communing with the spirits of the dead, almost always one's ancestors or departed loved ones, sometimes very exceptional or generally famous-in-their-own-time people who are now dead.

Only fairly recently has necromancy been expanded or altered to include or mean ... literally reconstituting a formally dead person's body, or raising a corpse from the grave, or something like that.

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Necromancy encompasses a broad range of rituals that are defined by the belief that whatever the ritual or rituals involved are, they will result in some kind of communication with or from a deceased person.

Osteomancy is a particular kind of ritual, defined by the method of manipulation of bones being a part of the ritual, usually throwing them on the ground and interpreting how they land, or a modern example would be seeing how a wishbone breaks in half to determine whose wish will come true.

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Necromancy does not necessarily, and historically did not always involve osteomancy.

You can be a necromancer, do necromancy, without throwing bones.

Osteomancy does not necessarily, and historically did not always involve, invoke or have the same goal, or explained functional mechanic as necromancy.

You can throw bones and divine a meaning without any part of any explanation of how this works involving the communication from dead ancestors.

You can break a wishbone and believe it indicates whose wish will come true without involving a dead person is making the bone break that way as a means of communicating to you or predicting the future.