First ask yourself: what is necromancy? So let's ask someone who knows: Saint Isidore. And he says (Etym. viii) in Greek, nekron "means dead and manteia divination, because after certain incantations and the sprinkling of blood, the dead seem to come to life, to divine and to answer questions."
So Necromancy is species of divination. Let's ignore how necromancy works exaclly it's by demons, always ans ask question that stated this thead: since necromancy is in it's very esence divination, is divination a sin?
Yes, for it is written (Deuteronomy 18:10-11): "Neither let there be found among you . . . any one that consulteth pythonic spirits, or fortune tellers": and it is stated in the Decretals (26, qu. v, can. Qui divinationes): "Those who seek for divinations shall be liable to a penance of five years' duration, according to the fixed grades of penance."
But we can speak more: Divination denotes a foretelling of the future. The future may be foreknown in two ways: first in its causes, secondly in itself.
Now the causes of the future are threefold: for some produce their effects, of necessity and always; and such like future effects can be foreknown and foretold with certainty, from considering their causes, even as astrologers foretell a coming eclipse.
Other causes produce their effects, 'not of necessity and always, but for the most part, yet they rarely fail: and from such like causes their future effects can be foreknown, not indeed with certainty, but by a kind of conjecture, even as astrologers by considering the stars can foreknow and foretell things concerning rains and droughts, and physicians, concerning health and death.
Again, other causes, considered in themselves, are indifferent; and this is chiefly the case in the rational powers, which stand in relation to opposites, according to the Philosopher [Metaph. viii, 2,5,8.] Such like effects, as also those which ensue from natural causes by chance and in the minority of instances, cannot be foreknown from a consideration of their causes, because these causes have no determinate inclination to produce these effects. 'Consequently such like effects cannot be foreknown unless they be considered in themselves.'
Now man cannot consider these effects in themselves except when they are present, as when he sees Socrates running or walking: the consideration of such things in themselves before they occur is proper to God, Who alone in His eternity sees the future as though it were present. Hence it is written (Isaiah 41:23): "Show the things that are to come hereafter, and we shall know that ye are gods." Therefore if anyone presume to foreknow or foretell such like future things by any means whatever, except by divine revelation, he manifestly usurps what belongs to God. It is for this reason that certain men are called divines: wherefore Isidore says (Etym. viii, 9): "They are called divines, as though they were full of God. For they pretend to be filled with the Godhead, and by a deceitful fraud they forecast the future to men."
Accordingly it is not called divination, if a man foretells things that happen of necessity, or in the majority of instances, for the like can be foreknown by human reason: nor again if anyone knows other contingent future things, through divine revelation: for then he does not divine, i.e. cause something divine, but rather receives something divine. Then only is a man said to divine, when he usurps to himself, in an undue manner, the foretelling of future events: and this is manifestly a sin. Consequently divination is always a sin; and for this reason Jerome says in his commentary on Micah 3:9, seqq. that "divination is always taken in an evil sense."
Necromancy is a sin because divination is a sin.