Having an entire chapter based in this region almost seems unrealistic.

Why is that? Lots of chapters are ancient organisations that dates back from millenia. Askellon wasn't necessarily THAT backwater then. They could also have been organised there because the Imperium thought there would be a lot more expansion in the region, and then, it was forgotten or the project of expanding the Imperium in this region stopped, leaving a Space Marine chapter homebased there.

Anyway, it's not as if Space Marines hadn't the means to go fight elsewhere if needs be.

You could always have a fleet based chapter (Carcharodon astra, Black Templars) coming trough the sector on a (penitent) crusade (Lamenters, Mantis warriors)

There is a reference to a space marine chapter in one of the planet write ups, it gives no name just a vague reference to the livery they wear and the fact that every generation or so they take a select few young warriors to join them. I don't have my books at hand so I can't remember which planet it is.

I could see a fleet based Chapter, coming through and looking for recruits. It would be interesting if chaos forces like Alpha Legion was trying to taint the recruiting process.

Having an entire chapter based in this region almost seems unrealistic.

Why is that? Lots of chapters are ancient organisations that dates back from millenia. Askellon wasn't necessarily THAT backwater then. They could also have been organised there because the Imperium thought there would be a lot more expansion in the region, and then, it was forgotten or the project of expanding the Imperium in this region stopped, leaving a Space Marine chapter homebased there.

Anyway, it's not as if Space Marines hadn't the means to go fight elsewhere if needs be.

Mostly my thought was that having any significant amount of space marines over this small of a sector for a sustained period of time would probably have cleaned up **** near everything short of the pandemonium by now.

Though a fleet based chapter would probably make more sense, recruiting in Askellon but operating in more important regions of space. Though one would think that any Chapter which does heavy recruiting in the sector would have a lot riding on the sectors safety, and would thus defend it a little better.

Mostly my thought was that having any significant amount of space marines over this small of a sector for a sustained period of time would probably have cleaned up **** near everything short of the pandemonium by now.

Because even if they were stationed in Askellon, this doesn't mean that Askellon would be their priority. If Askellon isn't the segmentum obscurum priority now, it means that the space marines stationned there would be called for other missions elsewhere.

I could see a fleet based chapter just passing through the area. Doing a tithe on a world that it will never see again.

About the fleash tearers idea.

I've ran the first half of my scenario tonight with my players.

They were traumatised and experienced the horror of marines turning blood frenzied one on another.

They saved a marine from the slaughter, which they followod into the battlebarge sitting silent in orbit. Their, they met the captain of the Flesh Tearers company, where he spoke to them about the curse of Sanguinius' Lineage.

At the end, my Blood Angel loving character just said "Can we help you..we could help you...we've got ressources, we know where the frenzied one are...we brought you back the progenoids..."

He reverently give them to the wounded captain.

The captain orders his remaining two "sane" marines. One goes to overload the plasma core and the other, to crash the battle barge on the ground because he learnt that his Archivist didn't find a cure on the surface of the planet.

The captain locks the door.

"No. Our secret will die with us. I'll at least give you a warrior death"

My player shoot through the head of the space marine pilot and one shot him, while my templar calix devoted to Sanguinius reluctantly fight the wounded captain (and just slush him).

It was a very intense game, with a lots of emotions and fun!

Ideas:

• The Acolytes purge some Daemons. The Grey Knights turn up. This is a bad thing.
• On Nurn Delta, the sky warriors have come for the heroic youths again, but they're apparently way off-schedule.
• A Deathwatch Kill-Team desperately needs the Acolytes to quietly place locator beacons for them, in various territories throughout the Hive.
• Logs on Enkidu, which was involved in the Nova Terra Interregnum, implicate the Dark Angels in something insidious.

Someone high up within the Adeptus Astra Telepathica notices that a lot of young, male psykers are going missing on their way to Earth. The Inquisitor the acolytes work for wants to know why.

It turns out to just be the Grey Knights recruiting them. But when the very existence of the Grey Knights is kept secret, investigating their recruitment process is going to be dangerous.

The Acolytes are on a planet, when a tithe for the sky warriors start. Except they find out these sky warriors are Alpha Legion.

However there must obviously be at least some Space Marine presence if we can recruit Deathwatch marines as reinforcement characters.

I'd assume that wherever you find Ordo Xenos Inquisitors of any rank you'll also find the occasional Deathwatch team.

As for Astartes in DH campaigns, I'm in the camp of keeping them as far away as possible. Unless they're allies, I'd probably never go beyond the lone Marine as an antagonist, simply due to how vastly more powerful they are (that, or give the Acolytes tons of plasma, power and melta weapons just to stand a fighting chance).