The story is Tales of the First: The Desert and is the follow on from Tales of the First: The Calling but - here's the thing, you don't have to have read The Calling to appreciate The Desert, but once you've read the later ficlet you will want to read the prelude. Sam has a gift with language which makes the tone of this work utterly unique. It is deliberately archaic in its language (and as Sam's beta reader on this I'm sure he won't mind me saying that it's not as easy as the finished product makes it seem!) and as a result reads like a page from history.
Tales of the First is set in the Slayerverse but rather than focusing on any of the characters we've come to know from the shows, Sam takes the opportunity to explore the lore, the roots of that universe. The First Slayer.
What are you waiting for? Shoo. Go. Read.