One Ring to Rule Them All

The Inscription on the Ring:

Ash nazg durbatulûk, ash nazg gimbatul,
Ash nazg thrakatulûk agh burzum-ishi krimpatul.

One ring to rule them all, one ring to find them,
One ring to bring them all and in the darkness bind them.

The lines describing the rings:

Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in their halls of stone,
Nine for Mortal Men doomed to die,
One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.

The One Ring was crafted by Dark Lord King Sauron himself in the fires of Mount Doom in the land of Mordor.

He purposefully made the One Ring to become the most powerful among the Rings of Power, enabling him to control the other kings and leaders who each own them. He is able to rule over the other Lords as long as he wears it.

He placed most of his powers on the Ring upon its creation, hence he became very dependent to it. Not wearing it, his powers are weak. But, as long as the One Ring continue to exist, Dark Lord Sauron existence remains.