The Ainur (singular: Ainu) are the immortal spirits existing before the Creation in J. R. R. Tolkien's fictional universe. These were the first beings made of the thought of Eru Ilúvatar. They were able to sing such beautiful music that the world was created from it.

Sauron is the title character and the primary antagonist, through the forging of the One Ring, of J. R. R. Tolkien's The Lord of the Rings, where he rules the land of Mordor and has the ambition of ruling the whole of Middle-earth. In the same work, he is identified as the "Necromancer" of Tolkien's earlier novel The Hobbit. The Silmarillion describes him as the chief lieutenant of the first Dark Lord, Morgoth. Tolkien noted that the Ainur, the "angelic" powers of his constructed myth, "were capable of many degrees of error and failing", but by far the worst was "the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron". Sauron appears most often as "the Eye", as if disembodied.