Tasha
Tasha’s path to greatness began when she was adopted by the arch-hag Baba Yaga, who named her Natasha. Tasha went on to create various spells, including Tasha’s Hideous Laughter, and her magic-fueled ambitions brought her into contact with demons and demon lords, which she subjugated and used against her enemies. On the Material Plane, she became known as Iggwilv the Witch Queen and wrote the Demonomicon of Iggwilv, the greatest of all treatises on the Abyss and its demonic inhabitants. In recent years, Tasha has sequestered herself in the Feywild, achieving incredible power and slowly turning into a Fey creature. Tasha has become Zybilna, archfey of the domain of Prismeer.
In short, Tasha/Natasha, Iggwilv, and Zybilna are three separate people, but also the same person.
Answering the Summons
When Zybilna received Alustriel Silverhand’s summons to combat Vecna, the archfey was sorely needed in Prismeer. As a compromise, and to honor Tasha’s friendship with Alustriel, Zybilna sent a version of herself from the past to Alustriel’s side. The Tasha who appears is the true, current version of Tasha, not one from the past or future