The History of the Caliph Vathek, by William Beckford
Beckford wrote Vathek in French in 1782, at a time when there were two fashionable crazes: the Gothic and the Oriental. Beckford married the two genres in a tale of an Arabian king gone wrong. His story was published in English under the claim that it was a story directly from the Arabic. The name Vathek sounds kind of German to me, but the name and the story are very very loosely based on a real person, the Abbasid caliph Al-Wathiq ibn Mutasim, who ruled in the 9th century.
Vathek is about 150 pages long, and has no chapters; it just goes from episode to episode without a break. (Much, I suppose, as Beckford wrote it--he claimed it only took him a few days and nights.) There isn't much in the way of character development.
The story is that Vathek is a powerful caliph with a bad temper (his angry glance can kill people!) and a large thirst for knowledge. A demon, Giaour, shows up and offers him knowledge and power if he will renounce Islam and worship him instead. Vathek happily does this; he commits blasphemy, sacrifices children to Giaour, and generally makes everyone miserable. His mother is expert in black magic and helps him out.
Vathek however has a very short attention span and keeps breaking his demonic contract, especially when he meets a girl. They recommit to Giaour together and travel to Hell to meet Eblis (another name for the devil, I think). For a short time they enjoy luxury and unlimited power, but within a few days it's all over and they fall into eternal torment.
I will admit to sort of skimming this story, which isn't all that great. You needn't bother unless you're really into the genre. A funny thing about my ebook copy--it's a direct scan of the original book, and for some reason the Vathek story is bundled with Johnson's Rasselas. Why? I do not know.