The evil Maia who serves Morgoth and later becomes Tolkien’s Dark Lord is Sauron.
Sauron begins Tolkien’s mythology as a powerful spirit, or Maia, associated with the Vala Aulë before turning to the service of Morgoth. He becomes one of Morgoth’s chief servants during the First Age. After Morgoth’s defeat, Sauron survives and eventually rises as the central enemy of the Second and Third Ages.
In the Second Age, Sauron helps create the Rings of Power while secretly forging the One Ring in the fires of Mount Doom. The Ring is designed to dominate the other rings and extend his control over their bearers. He later loses it when Isildur cuts it from his hand.
Sauron is sometimes confused with Saruman, who is a corrupted wizard, and with Morgoth, his earlier master. In The Lord of the Rings, Sauron’s influence is often represented by the Eye, although Tolkien’s full legendarium gives him a much longer history.