As one of the members of the Highborne, Lady Vashj considered it her birthright to serve Queen Azshara. Vashj copied the queen's hairstyle and clothing as much as possible. When the queen made Vashj her foremost handmaiden, remaining in the queen's favor became Vashj's sole concern.
War of the Ancients
When the Highborne were contacted by the titan Sargeras, Azshara was enthralled by Sargeras' vast might. Like the queen, Vashj looked forward to his arrival on Azeroth; both women were certain that he would find Azshara irresistible.
The queen did indeed possess amazing charisma, and she doubtless enhanced it with magic. With a quiet word or a mere glance, Azshara could inspire any number of violent emotions; passion and rage were her favorite choices. Driven by sudden outbreaks of jealousy, her subjects occasionally squabbled over her affections and fought duels in the name of her honor. The queen was amused by such displays and did not always intervene to save the combatants from harm.
It was precisely this sort of possessive fury that affected Vashj when Azshara and her handmaidens paid a visit to High Priestess Tyrande Whisperwind. Tyrande had been captured by the Burning Legion and imprisoned in the palace. On a whim, the queen offered to make Tyrande one of her handmaidens, but Tyrande politely declined, saying she was sworn to Elune before all others. Azshara was irritated by Tyrande's refusal, however diplomatic, and departed shortly thereafter.
Vashj could not forget the queen's offer so easily: the memory became a torment. Despite the fact that Tyrande had refused the queen, Vashj came to see the priestess as a rival for Azshara's affections. Irrationally certain that she had to act, Vashj tried to kill Tyrande, but failed. Vashj then fled the prison cell before her attempt on Tyrande's life could be discovered.
Cursed and Transformed
Despite the Highborne's attempts to aid the Legion in summoning Sargeras, the portal to the Twisting Nether was ultimately closed before Sargeras could reach Azeroth. The Well of Eternity was unable to withstand the added magical strain, and it began to collapse in on itself. Under the sudden impact, Zin-Azshari and the Well were blasted downward toward the ocean floor. The Well pierced the planet's molten core just before being utterly obliterated.
Ancient Kalimdor was split apart, and the seas rushed in to fill the voids between the new landmasses. Azshara, Vashj, and quite a few other Highborne were caught in the rushing waters. Instead of drowning, however, Vashj and many others were cursed and transformed into naga.
Despite her subsequent mutations, Vashj has managed to retain her memories and personality. She has also become immortal, and she is not the only naga to receive this unexpected boon.
Serving the Betrayer
Several months after the Third War, the demon lord Kil'jaeden went to Outland and contacted Illidan Stormrage. Although Illidan had worked against the Legion's interests in the past, Kil'jaeden was willing to give Illidan another chance. If Illidan destroyed the Frozen Throne, Kil'jaeden promised to grant anything Illidan might desire.
Accordingly, Illidan sought out new allies. He cast a powerful spell that reached the depths of the ocean, and a group of naga led by Lady Vashj responded. Vashj and many others remembered Illidan, who had become infamous before the Sundering. The naga respected his power, particularly now that Illidan had become a demon. They agreed to serve him.
Illidan and the naga retrieved a magical artifact known as the Eye of Sargeras, and they channeled a destructive spell through the Eye in order to destroy the Frozen Throne. However, their attempt was thwarted, and Illidan made a hasty retreat to Outland in the hope of evading Kil'jaeden's wrath.
At Illidan's direction, Lady Vashj and her naga sought out the Prince Kael'thas Sunstrider and his group of blood elves. The desperate prince at first hesitated to accept the naga's aid in fighting the Scourge, but practicality won out in the end. Then Kael'thas' human allies discovered the makeshift alliance, accused the blood elves of consorting with the enemy, and imprisoned them. Again Vashj and the naga came to the rescue and helped the blood elves escape the Dalaran dungeons.
The blood elves were suffering terribly from their hunger for magic by this time. Vashj explained that the naga, too, were addicted to magic, and she said that Illidan could help the blood elves. She then led them to the portal that Kel'Thuzad had opened during the Third War (so as to give Archimonde entry into Azeroth). With no other palatable choices, Kael'thas and his fighters followed Vashj to Outland. Together the blood elves and naga freed Illidan from Warden Maiev Shadowsong.
Kael'thas swore allegiance to Illidan, and he and Lady Vashj joined Illidan in defeating the pit lord Magtheridon and laying claim to his Black Temple. Yet Illidan and his allies did not celebrate their conquest for long. In a storm of magic and fury, Kil'jaeden descended upon Outland and demanded that Illidan once again try to destroy the Frozen Throne.
With no other option, Illidan led the naga and blood elves back to Azeroth. The battle came down to single combat between Illidan and the death knight Arthas Menethil. Arthas won the fight and warned Illidan to leave the planet and never return. Since that time, Illidan and his allies, including Lady Vashj, have inhabited the Black Temple on Outland.