C.L.Moore (the gender-erasing pen name of Catherine Lucille Moore) created Northwest Smith for the pulp Weird Tales. Smith is the template for Han Solo, a swashbuckling protagonist of suspect morality but ruled by an internal integrity and embued with a fundamental courage. In this most famous 1933 Northwest Smith story, the first story ever published by Moore, he rescues a mysterious woman (he almost always did) who turns out to be more and strangely less than she seems.
The secret of Shambleau is that she's not human, in fact humans are a source of sustenance for her. What kind of sustenance is the core secret of the story. Moore creates a wonderful and terrible synthesis of beguiling beauty and weird horror all in one exotic intoxicating package. If you've never read "Shambleau" you must. I don't want spoil it, but I fear I've said too much already.
Here is a link to the collection in which I most recently read the story. Below are some covers for editions which have included the tale, which has been widely reprinted.