This is an incorrect definition. "Necromancy" is not communication with the dead per se, but conjuring up a dead person's spirit for the purpose of fortunetelling, which is exactly what King Saul did in Scripture. (The "-mancy" ending refers to fortunetelling; cf bibliomancy, lithomancy, cephalomancy, etc.)
Conjuring up (or otherwise trying to achieve) a manifestation or visitation from a dead person is sinful, just as demanding or trying to coerce God into appearing or working some miracle would be sinful. The key here is the coercion or conjuring. Asking God for a miracle is not sinful. Trying to force him to perform one or demanding it is.
Fortune-telling of any kind is also a sin. Both are sins against the virtue of faith and the First Commandment.
It may be incomplete; it's certainly not incorrect.
Necro = dead
mancy, etymologically, means either forth telling something prophetically, or foretelling something prophetically.
See post #34 at: They see dead people?
Certainly what you've contributed here puts the proper focus on man trying to initiate such contact.