Sauron is an Ainu. The Ainur are a race of primordial spirits created by Eru's thought (Eru = creator of existence). They existed before/outside of Arda (Arda = the world)with Eru. Some were chosen to be placed in Arda and to govern it. The 14 ruling Ainur are the Valar ('the powers'). The rest are Maiar, servants of the Valar.
Sauron used to be a Maia. Melkor (a former Vala) was the first Dark Lord and corrupted Sauron, using him as his right-hand man. Melkor was tossed into the void. With his departure, Sauron became the Dark Lord.
Those of the Ainur race are immortal, thus Sauron is.
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...take counsel with thyself, and remember who and what thou art.
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