From the article:
Necromancy, or communication with the dead, has also played an important role in Mormon history, beginning with Smith's earliest "visions." Mormon leaders have traditionally taught that he was actually a spirit medium. For example, in an 1853 sermon, LDS elder Parley P. Pratt revealed that Mormonism is founded entirely on the practice of necromancy, and that the spiritualist movement of the Nineteenth Century, which had begun only five years earlier, actually aided the cause of the LDS church:Who communicated with our great Prophet, and revealed through him as a medium, the ancient history of a hemisphere, and the records of the ancient dead? Moroni, who had lived upon the earth 1400 years before....Who revealed to him the plan of redemption, and of exaltation for the dead who had died without the Gospel and the keys and preparations necessary for holy and perpetual converse with Jesus Christ, and with the spirits of just men made perfect?... Those from the dead!...Shall we, then, deny the principle, the philosophy, the fact of communication between worlds? No! verily no! Editors, statesmen, philosophers, priests, and lawyers, as well as the common people, began to advocate the principle of converse with the dead, by visions, divination, clairvoyance, knocking, and writing mediums, etc., etc. This spiritual philosophy of converse with the dead, once established by the labors, toils, sufferings, and martyrdom of its modern founders, and now embraced by a large portion of the learned world, show a triumph more rapid and complete — a victory more extensive, than has ever been achieved in the same length of time in our world.An important point is gained, a victory won, and a countless host of opposing powers vanquished, on one of the leading or fundamental truths of "Mormon" philosophy, viz. — "that the living may hear from the dead."Pratt was an Lds "apostle" -- and he's Mitt Romney's great-great grandfather. Here, he...
...labeled Joseph Smith a "medium."
...said it was "the dead" who communicated these things to him.
...said we can't deny the communication of the worlds (what Mormons call "the spirit world" with this world)
...he labels this worldview a "leading or fundamental truths of "Mormon" philosophy, viz. — "that the living may hear from the dead."
...and even labels this conversing with the dead, which he recognizes as being "advocated" "by visions, divination, clairvoyance, knocking, and writing mediums," "...show a triumph more rapid and complete — a victory more extensive, than has ever been achieved in the same length of time in our world.
Necromancy, or communication with the dead 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.