The dark lord who forged the One Ring in J.R.R. Tolkien’s mythology is Sauron.
Sauron created the One Ring in the fires of Mount Doom during the Second Age. He invested much of his power in it so that he could dominate the other Rings of Power and control their bearers.
Before becoming the principal enemy of The Lord of the Rings, Sauron served Morgoth, the first Dark Lord. After Morgoth’s defeat, Sauron continued his own campaign to control Middle-earth, eventually establishing the realm of Mordor.
Morgoth and Sauron are therefore related but not interchangeable answers. Morgoth is the earlier and more powerful master; Sauron is the enemy whose Ring drives the central conflict of Tolkien’s trilogy.