Eye of Anu
From Diablo Wiki
"Before the beginning there was void. Nothing. No flesh. No rock. No air. No beat. No light. No dark.
Nothing, save a single, perfect pearl.
Within that pearl dreamed a mighty, unfathomable spirtit-the One–Anu. Made of shining diamond. Anu was the sum of all things: good and evil, light and dark, physical and mystical, joy and sadness-all reflected across the crystalline facets of its form. And, within its eternal dream-state, Anu considered itself-all of its myriad facets. Seeking a state of total purity and perfection, Anu cast all evil from itself. All dissonance was gone.
...In the epicenter of reality lies Pandemonium, the scar of the universe's violent birth. At its chaotic center lay the Heart of Creation, a massive jewel unlike any other: the Eye of Anu - the Worldstone. It is the foundation stone of all places and times, a nexus of realities and vast, untold possibility."
For more information, see Worldstone.