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 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. Eventually, they encountered Eldrith and after killing her they too destroyed the Onyx 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 in Baldur's Gate. Mordoc was then in control of the Tower for a small while, but upon his death at the hands of adventurer's the Onyx Tower crumbled to dust for the last time.