The Eye of Balor was one of six jewels that the Goa'uld once used as power sources. Though it was originally owned by the System Lord Balor, it was lost upon his death over the planet Magtireth. Anubis's agents were able to find the Eye, however, and stole it from Morrigan's planet. (RPG: "Living Gods: Stargate System Lords")
Anubis later used the Eye to power the superweapon aboard his mothership. The Eye was destroyed when the Tau'ri managed to detonate the weapon's power core as Anubis attempted to destroy Vis Uban. (SG1: "Full Circle")