Since we're establishing that the Rising Sons have domain over an entire sector, we can have a shrine world as well as a feudal world and a WWII Japan world. Here's how I see the feudal world:
An entire planet modeled after Sengoku Japan, mostly clusters of island with few large land masses. The Rising Sons let the clan lords govern their own territories as long as they pay their taxes to the capital province, where the Shogun resides. They encourage war amongst the clans to breed the finest warriors.
During battles of special importance, the Shogun may send representatives to observe the battle. The clan generals will inform their troops that the Shogun's men are watching and to fight their hardest. The most outstanding of warriors will be called to serve in the Shogun's personal clan, where they will be trained to be space marines.
Occasionally, a clan lord will get over-confident and attempt to attack the planetary capitol and take the title of Shogun for himself. Of course, these attacks invariably end in failure, but it isn't uncommon for the Rising Sons to recruit from the clans of these failed attacks. If a man is brave enough to fight a space marine, imagine what he'll be willing to fight as a space marine!