The Crystal Arch is the heart of the High Heavens, the place where angels are sung into creation. The Crystal Arch is rumored to be the Spine of Anu, the primordial creature of good that was destroyed at the creation of the Universe.
In the third game, Diablo, as the Prime Evil, wields the combined powers of all seven Evils to shatter the Diamond Gates and storm the High Heavens, even ascending the Silver Spire and managing to corrupt the Crystal Arch, rendering all the Angels powerless. After he is cast down, however, the corruption of the Arch is reversed, and the Heavens are restored to their former glory and majesty.