As for the background, nothing is set in stone yet, but here’s what I’ve been kicking around:
The High Lords of Terra sanctioned the creation of this chapter primarily to protect a populated star system in the galactic South East. The system [needs a proper name] is strategically important to the Imperium because of the high rate at which its human inhabitants produce psychically attuned offspring. Estimates of psychic abilities in the system’s human population range from 1:50,000 to 1:10,000 These psykers are, of course, a very valuable resource to the Imperium of Man, and the Black Ships make frequent stops in the system to collect such specimens for use as agents of the Imperium, astropathic messengers, in one of the many psychic choirs, or even to fuel the Golden Throne itself.
Scientists have speculated that some strange combination of solar radiation and mineral composition of the planets' crust conspire to create the abnormally high rate of psychic mutation in the planet's inhabitants. One archeologist has even gone so far as to claim that the worlds of this system were once home to an ancient race of Xenos--perhaps the Eldar. Still there are others within the Imperium that suspect the taint of chaos must be close at hand where humans are frequently born with such gifts.
Regardless of the causes of these phenomena, the system remains sufficiently important to the Imperium that when an Ork waaagh began to mobilize within the sector, members of the Ultramarines defended the system from any and all attacks as the green tide washed over the neighboring stars. After the waaagh had passed. A garrison of Ultramarines remained behind with a compliment of strike cruisers, thunderhawk gunships, and a pair of dreadnoughts, to defend the system from further Ork incursions into the system. Occasionally, the Ultramarines even drew gifted young psykers from the local populace for training within the Librarium of Macragge.
Under the vigilant watch of the Ultramarines, the system prospered and grew from a scrabble of primitive mining colonies and feral worlds into a productive feudal society with a competent network of manufactorums and a capable Planetary Defense Force capable of lending its strength to the Imperial Guard when necessary.
During this time, gene seed from the Ultramarines was used to create a new chapter to permanently watch over this system, and to aid the Ultramarines in defending the Ultima Segmentum against the enemies of mankind. The High Lords dubbed this new chapter the Shadow Guardians. They are to guard the Emperor’s Feast for all eternity. Because the Shadow Guardians recruit primarily from worlds within the system they are sworn to protect, they boast a large number of powerful librarians within their ranks. Additionally, a few ordinary marines from the Shadow Guardians possess low-level psychic powers which might appear to others as a strong sense of battlefield intuition.
During the Time of Ending at the close of the 41st millennium, the Shadow Guardians find the worlds in their charge under heavy attack from the advancing tendrils of the Great Devourer...
This is really all I've thought out so far. I'm thinking that the chapter is currently under heavy attack from Hive Fleet Leviathan, or perhaps they have already been routed from the system by the endless waves of clawed monsters.