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, and also called the Cursed 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.
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 (maybe), 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 two canon chapters seems to be doing pretty well: the Black Dragons, who the Inquisition wants to erase in the same way as the Flame Falcons, and the Minotaurs, who are the Space Marine-killing headsmen of the High Lords of Terra that may not even be the original Chapter. (Well, three if you count the Lamenters, at least they're still alive...)
Several active traitor warbands, especially the Blood Gorgons are alive and kicking (even if their author's career isn't) and pretty awesome, too, but, you know, evil chaos men and stuff. They were declared Excommunicate traitoris by the Inquisition; it's not clear if they actively opposed the Imperium, decided to just go and do their own thing, or were (this is perhaps very slightly implied) unjustly implicated and swept up in the whole scandal of the disaster that was the "cursed" founding to begin with.
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, if at all, 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. Or perhaps, Belisarius Cawl was tinkering around.
On the tabletopEdit
Fan-made chapters may also use this Founding to justify some peculiar genetic quirk.
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, mutations are synonymous with the involvement of Chaos.
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). And even the Black Dragons get a ton of crap to dodge in exchange for the advantages of their mutations. Not least Dark Eldar wanting them for science experiments.