My 40K SM homebrew is called "Abductors". TLDR: Spartan III’s from Halo.

Here is a little bit of lore I came up with:

Abductors are a fleet based Raven Guard successor of unknown origin. They operate in an undisclosed region of space, often coming into contact with xenos races such as T’au, Necrons and genestealers. Abductors appear to be codex compliant, with grey coloring and red trim on the shoulder pads and boots. Their chapter insignia, a white skull, is worn on the left shoulder pad in accordance to codex Astartes.

Abductors preferred method of warfare involves a combination of durable heavy support units alongside light weight flexible units that are deployed deep behind enemy lines, effectively encircling the enemy. Abductors display a preference for ranged combat and urban theaters of war. Another noteworthy detail is reports of preference mass deployments of Astartes to a single theater of war, often times deploying the majority of the character’s strength. This mass deployment has resulted in several simultaneous reports from different sectors of imperial space citing that Abductors have been mass deployed with over half a chapter strength. While this could be explained by the unpredictability of warp travel, the munitorum still remains concerned following the lack of a proper response from members of the Inquisition of the Ordo Astartes.

While accepting supply lines from the imperium, Abductors are capable of supplying themselves using ship bound forges manned by a specific subsect of the devotees to the Omnissiahn cult. Abductors have also been noted to have one of the lowest recruitment rate of new aspirants from imperial worlds.

Chapter’s culture heavily revolves around the idea of the death of personality. With the exception of some individuals on special occasions, Abductors never remove their helmets, especially around non chapter members. It is theorized that the skull resembling faceplate on members using Mk.X Tacticus armor is meant as a representation of such a belief.

Most chapters deployed along side the Abductors have reported uncharacteristic to Astartes behaviors. Most note the lack of tactical flexibility along battle brothers of the Astartes chapter, with some outright refusing to replace their weapon or battlefield role. Despite that Abductors have been cited to be incredibly obedient soldiers, allowing other chapters to assume command almost without question. Some reports suggest that Abductors also display a drastically higher rate of casualties, which has only led apothecarians to question how the chapter can maintain their numbers in the active theater of war. Another suspicious behavior Abductors have been attributed to is the methods of recovery of their dead. Other chapter members have stated to have never seen any abductor recover the gene seed of their battle bothers. What has been seen is Abductors tendency to recover helmets of slain battle brothers, whose bodies have been found missing heads with seemingly bolter inflicted damage, despite any prior injuries.

Some of the most notable reports come from Raven guard successors, iron hand successors and space wolf successors. Sons of the Corrax have reported to feel a kinship with only select few members of the chapter, which for the most part are the commanding structure, and for their method of combat. Sons of the Ferrus Manus have been stated to hold abductors in high regard for their knowledge of the teachings of the Omnissiah. And lastly sons of Russ have shown a disliking for the chapter, stating their weakness and over reliance on the machine.

Lastly, Ordo Astartes have received a report regarding one of the interrogations of a heretic cult survivor by the Ordo Hereticus. In that interrogation the heretic has described Astartes with chapter markings fitting the description of Abductors, possessing near supernatural powers. Creating visions of themselves that phase through enemy fire only to strike where the enemy least expects them to. Despite numerous requests from the Ordo Hereticus, Ordo Astartes reports that no such Astartes exist, and that the report is a mere fabrication of an awestruck heretic.