21st Founding

One of the many Space Marine Chapter Foundings since the Legions were created for the Great Crusade. This one in particular is noteworthy because it was the largest since the Second Founding.

Fresh with the start of the 36th Millennium, five millennia after the First Founding, the Adeptus Mechanicus felt that it was about time to tinker with the gene-seed of the new Chapters and correct the many errors that had accumulated over the centuries.

They failed.

For one thing, many of the resulting Chapters had entirely new mutations introduced, like the Flame Falcons, who actually were covered in flames, until the Inquisition wiped them out.

Even when the Mechanicus managed to avoid creating mutated Chapters, it seems that their other creations were cursed with bad luck. For example, the Fire Hawks became the Legion of the Damned after they were lost in the Warp, and the Lamenters, who apparently managed to end up cured of the Black Rage and Red Thirst, are apparently the most disaster-prone Chapter in the Imperium, and then turned out to fall to the Black Rage anyway. Only one single canon chapter seems to be doing pretty well: the Black Dragons, and the Inquisition wants to erase them in the same way as the Flame Falcons.

Because of these many misfortunes, the 21st Founding has come to be known as the Cursed Founding. Those Chapters unfortunate enough to have been founded at this time are barely tolerated by the Inquisition (or outright not tolerated), and are mostly shunned by the rest of the Imperium. It's not exactly known why the 21st founding was cursed of all things, but it was probably Tzeentch getting his dose of lulz. Given the extensive genetic engineering involved, it is also possible that Fabius Bile was somehow involved.

On the tabletop

Games Workshop actually released rules for the different cursed founding chapters way back in 2004 during the transition between 3rd and 4th editions. The summary for them is found here.

Fan-made chapters may also use this Founding to justify some peculiar genetic quirk.

Alternative take

It's called the cursed founding, by apparently introducing more mutations and flaws. However, how the fuck is creating the Legion of The Damned a bad thing? The Black Dragons got a kickass Wolverine thing (more like Baraka), among many other actual improvements.

....Oh right, The Imperium hates new things

Actually, the Black Dragons and Legion of the Damned were a small amount of loyalist rarities in a large founding (the largest since the 2nd Founding). Although we're given only a small amount of specific examples, it's explicitly stated that the vast majority of Cursed Founding chapters fell to Chaos, went renegade, went INSANE, or had ridiculously bad luck and went extinct from it (the Lamenters apparently weren't the only ones. They're just the only ones that managed to survive in spite of it).