Wendonai

Wendonai was a balor lord who in 11,500 DR was tasked by Lolth with seducing the Sethomiir clan, rulers of the Ilythiiri, into her worship and granting them evil magics. He succeeded and was probably the greatest (but by no means the only) influence on the fall of the dark elves.
After the Descent, Wendonai continued his work, acting as an adviser, tutor, and sometimes consort to the most powerful drow Matron Mothers. He tried to claim a little power for himself by leading a successful drow and duergar attack against the Elven Court in 4400 DR, called the Dark Court Slaughter. Wendonai's efforts at personal growth over the next two millennia earned Lolth's ire as she had no intention to let the drow return to the surface. Lolth withdrew her favor from the demon in 2549 DR and the Matrons followed suit.
Wendonai then allied himself with Eltab, serving as his emissary to Faerûn for several centuries. In 623 DR, however, he was summoned by a Nar demonbinder but was soon banished by an incredibly powerful cleric of Horus-Re for 2000 years. The banishment was expected to end in 1377 DR and, after two millennia of planning, Wendonai looked forward to his return and his revenge against Mulhorand.
During the War of the Spider Queen, Lolth needed his services once more. He took possession of the Crescent Blade and lent it the strength it had lost after Halisstra Melarn broke the blade in the Demonweb Pits. Disguised as the blade of Eilistraee, he hoped to get to her most trusted priestess, Lady Qilué Veladorn. When Lolth sent the Lady Penitent back to Faerûn, she tricked the worshipers of Eilistraee into searching for the repaired blade in the Demonweb Pits. Qilué sent Cavatina Xarann and it was she who wielded the Crescent Blade against the demigod Selvetarm and killed him with the help of the balor within. After that, the blade and the demon came to Lady Veladorn. Wendonai began his work of corrupting the priestess. He tricked her into believing that she could kill Lolth with his help and he tried to possess her body but she resisted this for quite some time.
Meanwhile, Lolth gave the Lady Penitent to Wendonai and he sent her once more to Faerûn to bring him Cavatina Xarann. Cavatina resisted the demon and killed his body in the Abyss. But he wasn't truly dead, as his soul was still in the Crescent Blade. When Cavatina learned of this and where the blade was, she tried to talk with Qilué but the priestess trapped her because she had her own plans. She wanted to lure the demon into her body and drive him out with silverfire. With the final death of Wendonai she hoped to free all drow of the mark of Wendonai.
Qilué teleported to the only place where she thought the demon could be bound. The royal court of ancient Ilythiir where coronal Geirildin Sethomiir summoned Wendonai for the first time. Her sister Laeral Silverhand helped her but she did not want to kill Qilué so she magically stopped time for the corrupted Qilué and went to search for someone who knew about how to kill demons. When she was away, the Lady Penitent found the frozen priestess and took her to an ancient temple nearby. There she got hold of the Crescent Blade and tried to kill Qilué with it but Eilistraee took possession of Qilué's body (possibly as an Avatar) to talk sense into the Lady Penitent and free her from the clutches of Lolth. However Wendonai acted swiftly and tricked her into believing that it was Lolth disguised as Eilistraee. Furious, the Lady Penitent killed the goddess, and therefore she had done what Lolth and Wendonai planned all along for her (although their success would be later undone by the Dark Maiden's return, during the Second Sundering). When Halisstra Melarn was killed herself shortly thereafter, it left only Wendonai as the one-and-only Champion of Lolth. In truth, it was the balor's goal from the very beginning.



https://forgottenrealms.fandom.com/wiki/Wendonai