Rateliff maintains that Sauron first appears in the Tolkien legendarium as Sauron the Necromancer in the poem, The Lay of Leithian, and after the destruction of the tower at Tol Sirion, the Necromancer fled to Taur-na-Fuin, which he thought to be Mirkwood.
FORBES: Review: The Hobbit: An Unexpected Journey and The Mooreeffoc Effect