The Apple of Discord is a magical artifact created by the Goddess Eris to spark strife and dispute among others. After the Gods were stripped of their powers, the Elders sealed the apple in a vault in the Heavens.
After Noah became convinced that the council of Elders was corrupt, he orbed to the Heavens to stop them. His love Calliope had told him about an artifact in the heavenly vault that could help him, which was revealed to be the apple. Using its power, he influenced Kyle Brody and many other Whitelighters to revolt against the council and even turned some Elders against their own. The fight was then stopped by Paige Matthews, who orbed the apple from Noah and reversed its effects. She then orbed it back to the vault.