In J.R.R. Tolkien’s legendarium, the dark lord who created the One Ring is Sauron.
Sauron forged the One Ring in the fires of Mount Doom during the Second Age. He designed it to control the other Rings of Power and to strengthen his rule over Middle-earth. Although the Ring contains much of his power, Sauron is not physically trapped inside it; its destruction makes him permanently unable to regain his former strength.
Sauron originally served Morgoth, the first Dark Lord, before becoming the main enemy of The Lord of the Rings. He appears in different forms across Tolkien’s history, including as a powerful spirit and as the ruler represented by the Eye in the later story.
Readers sometimes confuse Sauron with Saruman, who is a wizard and a separate character. Morgoth is also distinct: he is Sauron’s former master and an earlier, greater enemy in Tolkien’s mythology.