Sauron was a powerful fallen Maia, intent on dominating Middle-earth. Having discovered how to make magical Rings of Power from the Elven-smith Celebrimbor in the Second Age, he put much of his own power into the One Ring, which he secretly forged in Mordor. Because the One Ring gave control, at least in principle (the Three Rings of the Elves being hidden from him) over all the other Rings of Power, it made Sauron far more powerful and dangerous than before.