Although there are a number of monsters in popular fiction that have also shown up in the Castlevania series, fewer "monsters" based on actual history have also been included. For a long while, Dracula (based on the real historical person Vlad "Tepes" Dracula) was the only historically-based monster included in the series (and he essentially got prime billing, the "and" of the credits, if you will). Considering that he's attributed as one of the true historical vampires (for his penchant for grisly murders), it was only a matter of time before another historical vampire was included as well.
Thus, with the release of Castlevania: Bloodlines, Konami brought in the other truly famous "vampire" from history: Erzsabet Bathory. Redubbed "Elizabeth Bartley" (which may just be a simple mistranslation from English to Japanese and back again), the woman became Dracula's niece (on his dead wife's side), was turned to a vampire, and became the main antagonist of Bloodlines.
Although she was only used in this one game, her inclusion did create a nice parallel for the main story of the series: she's a relation of the villain while one of the two heroes, John Morris is the son of Quincey Morris, one of the heroes of Bram Stoker's Dracula. Thus there are "bloodlines" on each side of the fight.
Once a powerful, if crazed, countess, Elizabeth Bartley took after her uncle, the mad vampire lord Dracula. Whether embraced by Dracula himself or turned later, the countess was tried for her crimes after being found feeding upon a young villager. Centuries later, Elizabeth was brought back from the dead by the Drolta Tzuntes, a witch who pledged her life to the countess.
Reborn, Elizabeth was intent was bringing back her uncle, Dracula. Her plan, instead of just casting a Black Mass Ritual (like so many of his other followers) was to essentially kick off a war. Initializing a plot to assassinate the Archduke Franz Ferdinand, the Crown Prince of Austria, Bartley essentially started the Great War (later called World War I after World War II was started). With so many dead and dying, Bartley was able to use necromancy powered by these souls to bring about the resurrection of Dracula.
What she also did, though, was garner the attention of two heroes, John Morris and Eric Lecard (although, she did also attract Eric's attention after she attacked, and embraced, Eric's betrothed -- that tends to turn anyone against you). The two heroes (who were also childhood friends) teamed up and ventured to Dracula's castle high up in the Borgo Pass. There, they fought through the lengthy opening section of the castle but, instead of finding Elizabeth or Dracula in the ruins of the keep, they instead faced off against an armored knight.
From there, the chase was on as the heroes followed Elizabeth, Dracula, and their path of destruction, which lead them from Romania through Germany (and a munitions factory), Italy, and Atlantis, all before finally ending up at Castle Properina for the final showdown. Venturing through the castle's halls and the many trapped designed by Elizabeth, the heroes eventually came face to face with the countess herself. She first sent out a decoy, Medusa, to fight for her, before taking on the heroes herself. Although she wielded powerful crystals that she could use to unleash powerful magic, in the end her abilities were no match for the power of the heroes.
Defeated, Elizabeth hoped that Dracula would defeat the heroes and find a way to revive her once more. Sadly, he two was bested by the heroes, ending Elizabeth's plan. Soon enough the Great War ended, but wherever she was, Elizabeth could have the last laugh as the repercussions of her evil were felt for decades to come (leading into World War II and the events of Castlevania: Portrait of Ruin).
The English and Japanese manuals had differing stories for Elizabeth's exact machinations and plots. We've taken the liberty of streamlining the two here for a more cohesive story.
Fighting Against Elizabeth:
Elizabeth's fight (at least in the full, proper Hard Mode) takes two forms. She starts off as a version of Medusa (what we can think of as her "demon form" if we line her up with proper Dracula fights). She appear in a corner and lash out at the hero in four ways: a linear flame attack that starts high before moving, in an arc, downwards; a double-helix flame attack that requires the hero to be in a safe position to avoid damage; a lashing tail attack along the ground; and crawling towards the hero and trying to melee them. Frankly, this form is easy to defeat so long as you know where to stand, and it shouldn't prove too complex to survive.
Her next form isn't really any worse. It's more like a glorified auto-scroller, honestly, just one that takes place in a single room. She'll appear, warping to the top of the screen, and summon balls of energy, four at a time, that will then circle the center of the room. Don't worry about these as they won't hurt you. Instead, focus on Elizabeth who will teleport into the room, summoning a fireball that will fly across the screen. Hit Elizabeth and one of the crystals will break. Do this four times and she'll fly back to the center of the room and repeat the process, over and over, until she's dead. It's, frankly, the opposite of thrilling.
As noted, Elizabeth Bartley is directly based on the insane countess Erzsabet Bathory. This insane noble woman had a taste for blood and is considered one of the most prolific serial killers in history. She's attributed with torturing and killing hundreds of young women (maybe upwards of 650, although that number, raised at her trial, is likely inflated) in a 25 year span before being put on trial for her crimes. She was also attributed with bathing in the blood of the young women, although that tale came after her death and, too, was likely false. Still, the vampiric tales stuck with her, making her one of history's most famous "vampires" (second only to Vlad "Tepes" Dracula, of course).
Drolta Tzuntes is Elizabeth's accomplice in the game being the one (in both the Japanese and English manuals) to revive the vampiress. However, she doesn't actually appear in the main part of the game. Further, she's never glimpsed again after this game, leading us to assume she realized the error of her ways. Presumably, after this, she went into a life of honor and good deeds (maybe opening up a frozen yogurt shop). We only mention her here, even, because the game's Drolta was clearly based on Dorottya Szentes, one of Bathory's real servants and one of three people tried and convicted as accomplices to Bathory's many crimes.
While Bloodlines is based on Bram Stoker's novel, Dracula, Elizabeth is not referenced in any way in the novel. Her inclusion is sole to add more historical details to the overall story.
Elizabeth in Popular Culture:
Although the Castlevania version of the character has not appeared outside of Bloodlines, her historical progenitor, Bathory, has been the subject of a number of books, comics, and movies, including the Hammer horror classic Countess Dracula and, much later, the direct-to-DVD flick Fright Night 2: New Blood (which no one would describe as a "classic").
And while she's not officially in other Castlevania games, fans have embraced Elizabeth as a worthy antagonist, such as in the fangame Silver Night Crusaders.