The Count of St. Germain was an adventurer in 18th century France who claimed to be an alchemist who was several centuries old. He was mentioned in a 1745 letter by Horace Walpole, and appeared several times in Casanova's memoirs. Several Theosophists claimed to have met St. Germain in the late 19th and early 20th centuries.
In 1978, the fantasy writer Chelsea Quinn Yarbro published a novel called Hôtel Transylvania in which St. Germain was actually a 3500-year-old vampire. Yarbro followed up Hôtel Transylvania with a series of other novels featuring the vampire St. Germain set in various historical periods, including ancient Rome in the time of the Emperor Nero, the courts of Ghenghis Khan and Ivan the Terrible, and France during the Black Death. These are technically horror novels, but the central conceit of the series is that all of the horrors portrayed are the work of ordinary human beings, and not of St. Germain and his fellow vampires.
In 2002, she published Night Blooming, featuring St. Germain in the Frankish Kingdom of Charlemagne. Thanks to one of the inexplicable quirks of the publishing industry, several hardcover copies of this novel wound up in the remainder bin at Shaw's supermarket in Middletown, Rhode Island last week, on sale for $4.95, about one fifth of the cover price. I happened to spot them there, and since I'm a fan of the series, I bought a copy. A quick search of Amazon.com shows that hardcover copies of this novel go for $3.18 and upwards (plus 3.99 shipping and handling), so thanks to the Shaw's supermarket chain, I have a copy on my bookshelf in perfect condition, without serious damage to my meagre checking account.