Vorpal Swords

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 worshippers as possible before they went down themselves.

It ended up being one of the biggest fails in Imperial history, as it was all a plot engineered by Chaos using the saint, who was a secret worshipper, to destroy or corrupt as many chapters as possible, weakening the Imperium while strengthening Chaos. The result was an increase in Chaos Warbands raiding the Imperium (and thus plently of opportunities for you to make your own new warband).

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 bought prove 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 before firing all his remains, the relics and his worshippers into the heart of a sun.

Since then the Vorpal Swords have served the Imperium once more, although that only shows they have a severe lack of brains, willingly serving the masters that were tricked by a single individual and caused the deaths of millions of innocent people and who in turn condemned them to a suicidal zerg rush. Thankfully with so little fluff on them you can make them out as having gone some common sense since then.