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 was so much against the city that her spirit lived on and it created the Onyx Tower in the middle of the Marsh of Chelimber. It also brought back the spirits of her comrades. Eldrith's and her comrade's life-force was 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 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. This time, Mordoc added a gate to the Onyx Tower entitled Mordoc's Gate, the gate allowed people to enter the Onyx Tower. Mordoc, like Eldrith, was connected to the Onyx Heart, an artifact which controlled the magic in the tower and was connected to the tower and the holder of the tower. With the death of Mordoc, the Onyx Tower had no holder and thus the Onyx Heart was destroyed, resulting in the tower's destruction.