Onyx Tower

The Onyx Tower was the most powerful creation on the face of Toril, it was ruled by the spirit of hatred, Eldrith the Betrayer. The Onyx Tower was created out of the Elemental Foundations and layed out on the Material Plane, it controlled all of the elements, this is what made it so powerful. It lay in the Marsh of Chelimber.

Origins
When Eldrith the Betrayer turned her back on the city of Baldur's Gate, archers chased her into the Marsh of Chelimber. In the marsh, they shot her down and killed her. However, her rage against the city was so much, that her spirit lived on and created the Onyx Tower in the middle of the Marsh of Chelimber. The Elemental Foundations, located in five of the planes in the Elemental realm, were activated. These foundations could only be shut with the destruction of the tower. In the Plane of Shadow, the Onyx Heart was built. The Heart contained the power to turn people into zombies and only with its destruction could the Onyx Tower be destroyed forever. The death of the Onyx Heart's holder could also destroy the tower. However, another person could reactivate the foundations and enter the tower and claim the Onyx Heart for their own. Doing so also brought back the spirits of her comrades, who were trapped in the tower. Eldrith and her comrade's life-forces were connected to the Onyx Tower and could only be freed with Eldrith's second death. Eldrith began to create Shadow Gates out of Onyx Material to create a Dark Alliance to destroy Baldur's Gate.

Eldrith's Alliance
Eldrith the Betrayer created a Dark Alliance to destroy the city of Baldur's Gate, however when her Alliance attacked three adventurer's they sought revenge. The adventurers travelled throughout Baldur's Gate, the Sunset Mountains and the Marsh of Chelimber and one by one destroyed the Dark Alliance. These three adventurer's, however, were not able to enter the Onyx Tower due to the fact it had no gate. Instead they had to go through the Elemental Foundation of Water to enter the tower and fight the Betrayer. In the tower they learned the secrets of the Elfsong Tavern and of the Betrayer herself; they learned much of the history of Baldur's Gate and of the Company of the Crescent Blade. The three adventurer's then fought with the Betrayer and managed to kill her in the top floor of the Onyx Tower. They then fled through the only remaining shadow gate, it's parallel was to be the only way into the tower. The Onyx Tower was then destroyed as Eldrith's death had caused the elemental foundations to shut down. The tower, however, still existed, it just did not stand, as the Onyx Heart had not been destroyed.

The Vampires
Mordoc SeLanmere then got control of the Onyx Heart and the only Shadow Gate that could lead back into the Onyx Tower once it was rebuilt. After the Harpers and Zhentarim reactivated the Elemental Foundations, the Onyx Tower was rebuilt just outside the Battle of Bones. When Mordoc was attacked, he moved it to Baldur's Gate. Using its magic he turned all of Baldur's Gate's citizen's into zombies and skeletons, minus soldiers who were patrolling outside the city, in the sewers and merchants such as Bartley who arrived after the tower. Mordoc's servant Xanhast turned certain citizens into vampires to patrol the streets during the short period while they had it, this included Randalla. Mordoc then connected himself to the Onyx Heart and entered the Plane of Shadow, redirecting the final Shadow Gate to transport its user there, as Mordoc planned to use the Heart's power to its full extent. Mordoc turned most of Baldur's Gate's citizens into zombies and with each citizen turned, the vampire gained more power. Mordoc was then apprehended by Vahn, Adrianna, Kromlech, Ysuran Auondril, Dorn Redbear, Allessia Faithhammer, Vhaidra Uoswiir and Borador Goldhand. The eight adventurer's killed Xanhast, who resided on the top of the Onyx Tower and guarded the final shadow gate. The eight then fought and killed Mordoc and the tower was destroyed again and the eight were trapped inside the Plane of Shadow. The eight then destroyed the Onyx Heart, this ensured the Onyx Tower was destroyed forever as now the elemental foundations could no longer be activated and the "heart" of the Onyx Tower had been destroyed, so in a way the tower itself was dead.