Boletaria

Background
During the twilight years of the Great Crusade a small contingent of the Dark Angels Legion, more specifically, the Gryphon Knights Chapter, under the command of Master Lordin Tolken, discovered a world previously overlooked by the Imperium. This planet, Boletaria, as it would become known, had a history akin to that of the Dark Angels' home of Caliban.

Boletaria’s human population had, after millennia of degradation during Old Night, had their technology reduced. They were also plagued by another misfortune - beasts afflicted by the Warp. During a particularly horrid Warp Storm near the planet, the wildlife of Boletaria was affected by the chaotic taint of the Immaterium whilst the now feudal humans had established cities in the shadow of great mountains, hidden from the Warp Storm's effects until it passed.

However, these Beasts would develop a maddened hunger for human flesh, chief among them being the horrible conjoining of a great feline and avian, as ancient Terran mythos would label as a "Gryphon," and assaulted the many mountain-fortresses of humanity across Boletaria. Thusly, orders of noble warrior hunters were formed after years of sustaining siege by savage monsters to slay the Beasts before they could threaten their respective homelands. And so, greatly intrigued by the coincidence of stumbling across another world like their own, the Chapter of the First Legion brought low the Beasts of Boletaria and began the process of bringing the planet into compliance, before the Great Heresy had begun, and the galaxy was split by the Ruinous Powers.

Horus Heresy
During the events of the Horus Heresy, the Gryphon Knights were stranded upon Boletaria and while most were uneasy about the state of the Warp, some found the familiarity of Boletaria to grow upon them. And many years later, when reunited with the rest of their Legion, and told of both the Emperor's fate, and that of Caliban and the Lion, the Gryphon Knights all took vows of shame that they were unable to lend their blades against the treachery of the former Warmaster's or Luther's conspirators.

When it came time to break up the Legions, the former Gryphon Knights parted from the Dark Angels and claimed Boletaria as their Chapter Homeworld, erecting the grand fortress of Basilica Drangleic upon the ruins of a fallen city of Boletaria known as Drangleic. They had recruited a fair number of recruits from the planet already, and the Knights were determined to not lose another home again. And to further keep the legacy of dead Caliban alive, the new Chapter maintained the patterns of Power Armor worn by the Legion during the Crusade, proudly keeping the traditions of the First alive in their own manner.

But thoughts of preservation would turn to vengeance as the knowledge of the surviving Fallen Angels reached the Sons of the Lion and the Unforgiven was formed, the Gryphon Knights became the Seraphim of the Abyss and merrily joined the Hunt to persecute and take vengeance for their first home lost, and their gene-sire's disappearance.

13th Black Crusade
When the wrath of the Despoiler was unleashed on the galaxy for the thirteenth time, Abaddon dispatched plague-ridden Space Hulks to the Scarus Sector, most of which were intercepted and destroyed by numerous Space Marine forces, but not all. As their pestilences wrecked havoc upon the Belis Corona Sub-sector, large numbers of Orks invaded the Sector, on their so called "Green Kroosade." This is where the Seraphim of the Abyss busied themselves as they were trapped in the Scarus Sector, unable to join the Unforgiven in their missions. But, the members of the Seraphim were content to lend their aid to the Sector that once held their home. However, the Chapter's prized hunter-killer tactics could not stem the tide of the greenskin hordes.

Though they were determined to aid the Imperial battle-groups sent to Scarus, the Seraphim of the Abyss were given the call to join the Dark Angels and the Unforgiven in the battle for Cadia, more specifically, to capture the Fallen among Abaddon's ranks. Filled with the fresh hatred and spite for what the Fallen had done, the Seraphim quietly left the Scarus Sector and joined the Unforgiven in their campaign, and were eventually present for Roboute Guilliman's reveal of the Primaris Marines to the hosts of the Unforgiven Chapters.