With Castlevania: Bloodlines, Konami took a very different approach to the storytelling than with other titles. Previous games had used the trappings of classic horror monsters to tell simple, Gothic horror adventures. Bloodlines, however, went the route of tying the Castlevania continuity not only to actual literature, Bram Stoker's Dracula, as well as actual history. Thus we have a story of Dracula and his "niece", Elizabeth Bartley (as in, Erzsabet Bathory), battling the son of Quincey Morris across Europe.
Working in more history, the instruction booklets for the game (both the U.S. and Japanese versions) allude to a servant of Elizabeth who aided the vampiress in resurrecting Dracula. This servant was based on the real servant of Bathory, Dorotya Semtesz, who aided the mad countess in the murder of countless young women. At the time of Bathory's own trial and punishment, Semtesz was one of only three servants also put on trial and executed as an accomplice. Thus, it's easy to see why Konami elected to add her to the back story for the game.
Oddly, although her part in the game is detailed in the instruction booklets, Drolta herself doesn't actively appear in the game; she's mentioned and then left out of the actual affair. It's a curious absence, one that makes you wonder if there had been bigger plans for the character that were cut during development.
In 1897 AD, Dracula was defeated at Whitby Manor at the hands of vampire hunters. His body was destroyed but his spirit was left restless. He needed a new form, and someone powerful enough to revive him. Thankfully (for him, but not for the world), there was one person who could aid him... if only they, too, were alive: his niece, Elizabeth. As it just so happened, though, a dark sorceress, Drolta Tzuentes, worked her magic to revive the dead niece, bringing the vampiress to full power. The two then hatched a plan to resurrect Dracula via the blood magic of the deaths of millions. Then then spent the next two decade fomenting a war, the Great War, to give them their magic.
While the world fell into war, Elizabeth set about having her revenge. She hunted down one of the descendants of the very vampire hunters that killed her uncle, a girl named Lucy Seward, and killed her just for the fun of it. Upon the discovery of Lucy, his betrothed, young Eric Lecarde went to his family's ancestral estate and grabbed up the family spear, vowing to kill Elizabeth and all that aided her. Meanwhile, in Germany, John Morris fought the forced of the dead that were rising. Realizing this was a sign of the growing power of Dracula, he set forth to the grounds of Castle Dracula to take out the evil vampire. He met up with Eric outside the castle and the two elected to join forces to fight the vampire threat.
Inside, though, they found not Dracula or Elizabeth or even Drolta but, instead, one of Drolta's mechanical monsters, the Mecha-Knight. The two had to battle to beast to destroy it and dispel the sorceress's power. They then ventured onwards, chasing the vampiress across Europe so they could put an end to this whole affair.
As noted, Drolta is not actually featured in the game itself, only its instruction booklet. It was thought that maybe Drolta was supposed to be in the game but was swapped out for a different boss, such as Death. Some have even speculated that Drolta was Death, under a guise, corrupting Elizabeth's mind to do evil. That would explain why Drolta isn't in the game. Of course, none of this speculation was ever confirmed by Konami.
In fact, it's not even guaranteed that she dies during the events of Bloodlines. Likely she does, but as the game never references her, and she doesn't appear in any later game, it's possible she survived and went off elsewhere. Maybe she took up knitting and opened a cat cafe. We simply don't know.