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You may cast Necromancy as though it had flash. If you cast it any time a sorcery couldn't have been cast, the controller of the permanent it becomes sacrifices it at the beginning of the next cleanup step.
When Necromancy enters the battlefield, if it's on the battlefield, it becomes an Aura with "enchant creature put onto the battlefield with Necromancy." Put target creature card from a graveyard onto the battlefield under your control and attach Necromancy to it. When Necromancy leaves the battlefield, that creature's controller sacrifices it.
The sacrifice occurs only if you cast it using its own ability. If you cast it using some other effect (for instance, if it gained flash from Vedalken Orrery), then it won’t be sacrificed.
If the creature card put onto the battlefield has protection from black (or anything that prevents this from legally being attached), this won’t be able to attach to it. Then this will go to the graveyard as a state-based action, causing the creature to be sacrificed.
Necromancy enters the battlefield as an enchantment and then becomes an Enchant Creature Aura as a triggered ability upon entering the battlefield. It follows all the rules for Auras from then on.
The bringing of the creature onto the battlefield and then putting Necromancy on it is all done as part of the resolution.
When putting a card onto the battlefield that requires a definition for its value or some other choice, you do what is needed to define the value or make the choice.
