"Swordsman of Lost Terra," a novella by Poul Anderson, fools you a little at the beginning by making you think its a straight fantasy. It's set on a version of Earth which doesn't rotate and has a Medieval level civilization and involves some apparently magical bagpipes that--if played just right--can instill panic in attacking enemies.
In fact, at first it's only the story's title that tells us the planet is Earth at all. But if we find out that it was originally published in the November 1951 issue of Planet Stories, then we have our first clue that it is in fact science fiction. Not hard SF--Planet Stories was not terribly concerned about getting the science right--but science fiction all the same. Those darn bagpipes are an artifact of lost science from before the world stopped spinning and old civilizations collapsed.
But mostly the characters in "Swordsman of Lost Terra" are concerned with hacking one another to death. The reader travels with what is essentially a nomadic warrior band--wandering around the Twilight area of the world until their homeland recovers from a famine. They make a living either hiring out as mercenaries or plundering what they need from others. They remind me a little of Vikings, which is not surprising in a Poul Anderson story. The man loved his Vikings.
There are three clans joined to form the band. Their overall leader is Red Bram, who is a pretty impressive kick-butt warrior. One of the other clan leaders is Rhiach, who is the only person able to play the god-pipe that brings madness or even death. Rhiach's son Kery--also a skilled warrior--is the story's main character.
When the band is attacked by a horde of well-armed, disciplined warriors from the dark side of the planet, it is only Rhiach's well-timed use of the pipes that saves them. But Rhiach is killed by a stray arrow. Kery inherits the pipes, but he has not yet been trained to use them. Now no one lives who can teach him.
All this is setting up a lively and exciting tale. Anderson puts his remarkable skill at world-building to work here, creating an Earth that in many ways has become an alien world compared to what we know. Then he manages to smoothly fit quite a lot of plot exposition and several epic battle scenes into the novella, along with several engaging characters.
The band soon allies itself with a city that is under siege from the Dark Lander army. There's a beautiful queen who falls in love with Kery, bloody battles, betrayals, kidnappings and desperate escapes. Everything revolves around the god-pipes. If either Kery or the leader of the Dark Landers can figure out who to use them, then the balance of power will rather abruptly shift.
"Swordsman of Lost Terra" is an excellent example of Sword-and-Planet fiction. It's really too bad Anderson never returned to this particular world. There are, I think, a lot of cool stories left to tell about it.
By the way, I read this story in a long-out-of-print anthology titled Swordsmen in the Skyi. I recommend this highly--keep an eye peeled for it in used book stores or online.