The Governor of the planet is handed a secret messege by a tech priest. Everone is overworked in the palace of the Governor and highly agitated. The Governor reads the following messege and..
--Imperial news network--
Ork Invasion has hit Klauze III. The Planet has held for a remarkable 2 month invasion against the invasion by a Orc Warlord named Deap Blak. Casualties are unknown due to ongoing conflict and danger. Estimates at 10,000 on the first month proved vastly understated and have not been updated or improved. Planetary PDF has announced that they are left with only 10% of original forces and relying heavily on militias, merceanaries, and Imperial Guard reinforcements. Imperial Guard regiments on the planet have been lost in the conflict due to loss of command, desertion, suacide of generals, and are throughly scattered with no communications to utilize.
More Imperial Guard Regiments have been sent and a request for help from a space marine chapter is being registered.
Signed,
Lazlo of division 4562 division of the Adeptus Ministorium
--End Messege--
...prays for help from the Emperor. Before he goes to sleep that night he thinks about what Space Marine chapter would help his crumbeling defenses repel the Orcs before the inquisition arrives for a final solution..
((Right then above is the story and below I imagine some warhammer 40k geeks enthusiasts like myself will post a chapter to come help this world. You can create your very own unique chapter or choose an already existing chapter such as the Blood Angels, Ultramarines, Space Wolves, etc.
Inside a spoiler you'll detail how many men your chapter has and if it follows the Codex Astartes. Each chapter will have 1000 men in total. Chapters will have 10 companies. Companies will have 100 men.
I would go on to say that the First company is to be considered your veteran company or terminator company, your tenth company to be a scout company, and for anything from 2-9 being assaults or whatnot. That said, if you create a space marine chapter that says screw those rules then you can create your internal organization differently.
The only thing you cannot deviate from is your 1000 man limit and the 1000 point rule. I'll allow some fudging if it's above only like 100 or so if it helps with authorizing your chapter. Other than that have fun guys i'll be back later today to pick a marine chapter.))
EDIT: Forgot to mention that the planet is a hive world, which for those that don't know is a big human farm for the imperial guard. Lots of people but ramapant corruption, crime, and at times herecy. That said there will be no daeminc elements in the game (unless I change my mind) and the focus will be you and the AI vs the Orks in a rng.