The evil spirit who becomes the Necromancer in J.R.R. Tolkien’s The Hobbit is Sauron.
In The Hobbit, the Necromancer is initially presented as a mysterious power occupying Dol Guldur in southern Mirkwood. The book does not immediately name him, but later Tolkien’s mythology identifies the figure as Sauron, the Maia who served Morgoth and eventually became the chief enemy of the Second and Third Ages.
The White Council drives the Necromancer from Dol Guldur near the end of The Hobbit’s chronology. Sauron then returns to Mordor and begins rebuilding his power, leading into the central conflict of The Lord of the Rings.
Saruman and the Witch-king are powerful servants or allies of Sauron, not the Necromancer himself. Morgoth is an earlier and greater dark power whom Sauron served during the First Age.