The Crystal Arch is the power source for all of the Angels of the High Heavens. All light eminates from it, and the arch is the most holy place in the universe. When all the Angels are in perfect harmony with the Arch's gentle music, an Angel may be born from it. If the powers of the arch are corrupted, all the angels are rendered helpless with no wings. But, if the arch topples, the High Heavens will fall, along with the Angiris Council and the entire Heavenly Host with it.
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. The Lord of Terror even ascends to the Silver Spire and manages 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 it's former glory and majesty.
During the battle with Izual, an angel ponders why Izual's replacement never arose from the Crystal Arch. This being because Izual's spirit is still bound to the Burning Hells. In other words, it is clear that there is a fixed number of angels, with each angel embodying an aspect of Anu, crowned by the Angiris Council, who define his 5 prime virtues. Only when an angel is killed, will the Crystal Arch produce his replacement, to restore that lost aspect of Anu. On which the number of angels are defined.