The dark lord opposed by the heroes in J.R.R. Tolkien’s The Lord of the Rings is Sauron.
Sauron is the principal enemy of the Third Age story. He seeks the One Ring because its power would allow him to dominate the other Rings of Power and impose control over Middle-earth. Although he does not appear as an ordinary character moving through the whole narrative, his influence shapes the political and military crisis surrounding the quest.
Sauron originally served Morgoth, the greater dark power of the First Age. After Morgoth’s defeat, Sauron continued his own program of domination. He helped create the Rings of Power and secretly forged the One Ring in the fires of Mount Doom, linking much of his power to it.
A common mix-up is treating Sauron and Morgoth as interchangeable villains. Morgoth is the earlier and more powerful master; Sauron is his former servant and the central enemy of The Lord of the Rings. Sauron’s physical form changes across Tolkien’s history, but his goal remains consistent: ordered control achieved through fear and force.