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 laid out on the Material Plane , it controlled all of the elements which is what made it so powerful . It was erected in the Marsh of Chelimber . History . 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 strong 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 down by destroying the tower . In the Plane of Shadow , the Onyx Heart was built . The Heart contained the power to turn people into zombies and only by 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 Eldrith ' s 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 . The Dark Alliance . Eldrith ' s Alliance . Eldrith the Betrayer succeeded in creating a Dark Alliance to destroy the city of Baldur ' s Gate , however when her Alliance attacked three adventurers , 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 adventurers 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 adventurers 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 thus the Shadow Gate that 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 . The multitude of undead that Mordoc commanded caused the vampire to gain 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 . The eight however , were trapped inside the Plane of Shadow . The eight then destroyed the Onyx Heart which ensured the Onyx Tower was destroyed forever as now the elemental foundations could no longer be activated .