Crystal Arch

"It is older than time itself, and over the eons this shining realm took shape around it. It thrums with living light and sound that courses through the very fabric of the Silver City."

- Tyrael on the Crystal Arch

The Crystal Arch is the heart of the High Heavens. Located atop the Silver Spire, it is the birthplace of angels. It is guarded by the Luminarei.

Lore
Mystics have described the Arch as being composed of uncountable diamond facets that shimmer with a brilliance indescribable in mortal words.

The Arch is said to be the spine of the creator god Anu, and to hum with a remnant of its essence that permeates all of Heaven. Many in Sanctuary, from the mystics of Aranoch to the priests of Skovos, agree that an echo of Heaven can be found at the height of meditation. Deckard Cain speculated that this echo might be the harmony that comes from the arch itself.

During Diablo's invasion of the High Heavens, he specifically sought to corrupt the Arch. With the fall of the Arch, all of Heaven would fall with it. And reach it he did, and the adverse effects began, as a wave of energy shot out, incapacitating nearby angels and causing their wings to fade. Tyrael (now mortal) and his mortal allies were immune to the effects however, and the Nephalem successfully defeated the Prime Evil. With his fall, the Arch shone once more, cleansing Heaven of Diablo's corruption.

Lightsong
New angels can be created through a process called the "Lightsong," otherwise referred to as the "Song of the Arch" or "angelic resonance." Here, angels tune their essences into the Crystal Arch in synch. Luminous bands flare off the Arch in perfect synchronization with its music, occasionally manifesting as new angels. These births are said to only occur during moments of perfect harmony in Heaven.

The amount of angels the Lightsong can give birth to is finite&mdash;it is a case of them embodying a set amount of Anu's power. If an angel is killed, a replacement angel will be created by the Arch, but such an angel is still a different individual from the one (s)he is replacing. The replacement does not come instantaneously. Of note, the Arch specifically called on Tyrael to take the mantle of wisdom after Diablo's defeat, taking the place of the absent Malthael.

The angelic resonance of the arch can be sensed by some humans. However, no human has been able to mimic it.

Weaponry and other equipment may be blessed by the Song of the Arch. Examples include the swords Azurewrath and Amberwing. Weapons forged in harmony with the Lightsong represent the greatest potential of angelic weaponry.

In-game
The Arch appears in Act IV of Diablo III. The map name appears as "The Crystal Arch", and it is where the final battle against Diablo takes place in the game. As befitting its supposed origin, the parts of the Arch that can be seen look like spurs from a giant vertebra made of glowing stone. As the battle rages, the Arch becomes more corrupted, its light blocked by growths of dark matter.