World of Warcraft
Xal'atath

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Xal'atath, the Harbinger of the Void, is an ancient and malevolent being of the Void, able to manifest in any form she chooses. She is both lethal and charismatic.
Once the Harbinger of the Void Lord Dimensius, she sought to free herself from his service and directed the Ravel into using the Reshii Ribbons, thus destroying their doomed world and Dimensius as well during the Devouring War.
Eventually, Xal'atath came to Azeroth, seeking to claim its worldsoul for herself. However, she was defeated and trapped by the Old Gods inside the Blade of the Black Empire, a Void-attuned sacrificial dagger. The dagger and the entity inside it passed through many hands throughout history, sowing death and chaos across Azeroth by whispering to and manipulating her wielders. She was involved with causing the ancient war between trolls and aqir, with Modgud's curse on Grim Batol during the War of the Three Hammers, and with Natalie Seline's study of the Void during the Second War.
A shadow priest adventurer wielded Xal'atath during the third invasion of the Burning Legion, before it was used to drain the Sword of Sargeras and was thereafter discarded, like the other artifact weapons. Xal'atath didn't die, however, and released herself from the blade during the Fourth War by possessing the corpse of the high elf Inanis, before concluding a pact with N'Zoth granting her freedom. She subsequently allied herself with the Primal Incarnate Iridikron, who gave her the Dark Heart, and assumed the role of Harbinger of the Void to usher in a new era of the Void's machinations.
Xal'atath then conscripted the nerubians of Azj-Kahet, as well as other inhabitants of Khaz Algar, into an army to claim the Worldsoul of Azeroth. Her destruction of the mage city of Dalaran started the war in Khaz Algar, which culminated in the battle of Hallowfall, where the Dark Heart was damaged.