Tear of Boccob
The Tear of Boccob was a tear-shaped crystal given to the archpriests of the Temple of the Mind's Eye in Estorium by Boccob in the year 1632. An order of priests, known as the Kaorti, was created to study the artifact. Their research led to many magical innovations, leading to an era of magical prosperity for Estorium. After the Illithid War, the surviving members of the Kaorti took the Tear out of Estorium, founding the town of Wellspring far to the north. Fearing what might happen if the illithids found the Tear, they constructed a hidden tower in the mountains near Wellspring to house it, and sequestered themselves there. Contact between Wellspring and the tower was limited and quickly ceased altogether. Eventually, the tower's location was forgotten and the tear fell into myth.
In the year 2959, when Tephra opened a portal to the Far Realm within the Temple of the Mind's Eye, the now-mortal Boccob sealed the portal and bound its energy to the Tear. However, in doing so, the Tear became corrupted, as the abberant energies of the Far Realm began leaking through it.
Tephra hired a mercenary band called the Stormcrows to reclaim the Tear in order to unseal the portal. When they located it, however, the Tear's corrupted energies warped them into hideous aberrations. They began worshiping it, until it was stolen by an illithid named Shathrax. Using his own mercenaries - a band of dark ones from the Shadowfell - Shathrax had various citizens of Wellspring kidnapped and brought to his lair in the sewers beneath the city. There he exposed them to the Tear, turning them into monsters.
It was around this time that the Challengers of the Unspeakable became aware of the Tear, and its interest to Tephra. Confiding in Dierdre, a priestess of Ioun in Wellspring, that they had learned of its possession by Shathrax, they delved into the sewers to reclaim it. Dierdre, meanwhile, took matters into her own hands, and used her magic to summon the Tear from Shathrax's clutches. Its corruption proved greater than she anticipated, however, and when she attempted to cleanse it her body was twisted and warped. Shortly thereafter, agents of Tephra stormed the temple and stole the Tear, taking it through the Shadowfell. When the Challengers returned, Dierdre confessed what she had done and urged them to destroy the Tear, as its corruption was too great.
Tephra's followers delivered the Tear to her at the Fortress at World's End in the Frostland, where she quickly set to work using it to open a small portal to the Far Realm. Using a specially-constructed orrery, she channeled its energy back through the Far realm to unseal the portal in Estorium. When the Challengers arrived, Kalth removed the Tear and threw it into the portal; the Far Realm energy spilling through the Tear into itself caused a planar feedback loop, and the Tear imploded.