Corrupting Light

"The other Diamonds were furious. In a last-ditch attempt to wipe out the Rebellion, the Diamonds launched a direct attack against Earth, and in their fury, they used their powers to end the fighting once and for all."

The Corrupting Light was a cataclysmic event and final assault against the Crystal Gems by the remaining members of the Great Diamond Authority. The attack resulted in the corruption of almost all Gems fighting on Earth and marked the end of the Rebellion.

While intended to only be used against the Crystal Gems, some Homeworld Gems, such as Centipeetle and her crew, were also caught in the attack because they were stranded on Earth when it was unleashed.

The light was first shown in "Same Old World" and later seen in the episode "Monster Reunion" in greater detail. The light is composed of blue, white, and yellow rays, the colors of the three current Diamonds.

History
At the end of the Rebellion, the Diamonds faced two massive losses: the "destruction" of one of their own (Pink Diamond) and the loss of her colony, Earth. In retaliation, the Diamonds combined their powers and launched a direct attack against Earth; the Corrupting Light.

Before the Diamonds attacked, Homeworld Gems were directed to leave the planet by one or more of their commanders. This caused a panic in which Gems fled the planet en masse in both spaceships and through the Galaxy Warp to Homeworld. In this commotion, Lapis Lazuli, who was in the mirror containing her, was dropped and stepped on, cracking her gemstone. Being poofed and trapped inside the mirror, she was saved from being corrupted.

After the majority of Homeworld Gems had fled, a light appeared in the sky, accompanied by what Steven interprets as "A sound? A song?". Later, in "Monster Reunion", the purpose of the light is explained by Centipeetle. It is confirmed to be the source of her corruption. It was most likely to ensure the Rebellion could never threaten Homeworld again. Based on her recollection of the event, it seems that the majority of Gems were not aware of what exactly they were fleeing from, as she was kept in the dark by her commander.

Trivia

 * The song in question, in addition to appearing in "Same Old World" and "Monster Reunion", has also been heard as a motif at the beginning of "Blue Diamond" and "Yellow Diamond".
 * The degree of the light and song's direct role in inducing corruption, or if they were merely byproducts of something else, is unknown.
 * The mental corruption caused by this light is referred to as "damage from the Diamonds" in "Monster Reunion" by both Pearl and Steven.
 * The following Gems were on Earth during the attack, but were not corrupted for various reasons:
 * Rose Quartz, Garnet, and Pearl (protected by Rose's shield)
 * Amethyst (presumably because she was still incubating in the Kindergarten)
 * Lapis Lazuli (due to being trapped in the mirror)
 * Bismuth (due to being bubbled and later being stored in Lion)
 * It was revealed in "Your Mother and Mine" that the corrupting light is a result of a direct attack from the three remaining Diamonds.
 * It is revealed in the trailer for the episode following "Reunited" the Diamond's attack wasn't intended to corrupt all gems on Earth, but obliterate all gems on Earth.
 * In order for a Gem to become corrupted, their physical form needed to be exposed to the light itself.
 * Lapis was exposed to the light but, because she lacked a physical form, she was not corrupted.