So, I'm in the early stages of planning a mission where the PCs oppose a group trying to raid artifacts hidden in the deep tunnels beneath the now-lifeless surface of the Yu'Vath capitol world. I was leaning toward using the Threnos Zone down in the Drusis Marches- the constantly-broadcast Imperial hymns that impair navigation would make perfect sense for this, and the next closest world (88 Tanstar) has a large Naval presence- also logical. The only problem is it's (relatively) close proximity to Sacris, the Homeworld of the Storm Wardens Chapter. Since the SWs predate the founding of the Calixis Sector, would it be at all feasable that a Space Marine Homeworld would be adjacent to the capitol of a Warp-worshipping xenos empire?

If I can't come up with a rational for this problem, I'll go with Xerios Prime in the Malfian Subsector- it has a nice, isolated possition- but I'd rather use the more colorful Threnos Zone. Are there any other examples of SM Homeworlds existing in the shadow of major xeno worlds?

Easily done, just add a 5,000+ year old warp storm to the space between systems that finally faded out over the last millenia or so, and had previously blocked any possibility of the them even knowing about each other before the Angevin Crusade.

Maybe the Storm Wardens established their home world there to guard this capitol world, to make sure the Yu'vath would never rise again?

Tie it in with the lore of the Storm Wardens, they have some kind of dark history too.