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.
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 somehow managed to end up cured of the Black Rage and Red Thirst, are apparently the most disaster-prone Chapter in the Imperium.
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.
Fan-made chapters may use this Founding to justify some peculiar genetic quirk, like the Knights Inductor.