The Crystal Arch is the remains of the spine of Anu after its death in the fight against Tathamet. The Arch became the foundation of the High Heavens, and it is from this arch that all angels, beings of light and sound, are born.
The Arch is located atop the Silver Spire, the tallest tower in the High Heavens. There it hums in a remnant chorus from Anu, and it is from this Arch that angels are born. It is unclear at what times this happens, but it appears to happen only when Heaven is in "perfect harmony".
In Diablo III, Diablo, 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.
- Cain, Deckard., Book of Cain, page 54.