Dungeons and Dragons
The "vorpal sword" was the weapon used to kill the monster in the famous poem Jabberwocky by Lewis Carroll of Alice in Wonderland fame, a poem which every English student had to study. It's also one of the most iconic weapons in D&D, a magic weapon that generally always caused instant decapitation on a critical hit. Usually, but not always, an instant kill.
|Allegiance||Imperium of Man|
|Colours||Light Green, with Black Pauldrons.|
The Vorpal Swords are a loyalist chapter of Space Marines who were part of the ill-fated Judged, a group of chapters who had been accused of heresy by the Imperial Saint Basillius the Elder. He gave them the choice of a swift death or taking part in the Abyssal Crusade, a lethal crusade into the Eye of Terror to kill as many chaos worshipers as possible before they went down themselves.
The Crusade ended up being one of the biggest fails in Imperial history, as it was all a plot engineered by Chaos using Basillius, who was secretly in league with the Chaos Gods, to destroy or corrupt as many Space Marine chapters as possible. This would offer the twofold benefit of weakening the Imperium's hold on the galaxy and potentially strengthening the forces of chaos. The resulting increase in the number of marauding Chaos warbands provides a great opportunity for you to come up with one of your own.
The Vorpal Swords were the only chapter (along with a few stragglers from some of the other chapters) to escape the Eye of Terror after the crusade. They brought proof of the saint's corruption to the Inquisition, which caused one of the biggest u-turns in Imperial history as every mention of the saint was redacted. His remains, his relics and his worshippers were then fired into the heart of a sun.
Since then the Vorpal Swords have served the Imperium once more, which is remarkable when you consider how thoroughly they were screwed over in the first place. That said, this could play into their fluff in some interesting ways. Surely they must be by far the most mistrustful of the Ecclesiarchy and the Inquisition after their experiences with them.
Most recently, they have had engagements against Chaos Daemons, including an (ultimately phyrric) victory against Skarbrand.