Apparently it's a self-contained game in the spirit of the old Necromunda where Space Marines - specifically the Blood Angels - square off against Orks in the hive cities of Armageddon.
While the default kill teams for the set are Orks and Blood Angels, apparently you can use different races for the two squads, which include but are not limited to:
Yeah...wait, what? CADIAN Infantry Squad? As much as I love Cadians, and I have 2 Veteran squads (well, they're Draconians, not Cadians), why would Cadians be on Armageddon? Where is the Armageddon Steel Legion? Are we supposed to pretend that the Cadians are Armageddon Steel Legion troopers or substitute them as such because the latter doesn't have models? What about Yarrick? This is Armageddon, right? It ain't Armageddon without Yarrick.
For those of you who are wondering why I haven't mentioned Shadow War: Armageddon until now, it's because when I saw Blood Angels vs Orks instead of Armageddon Steel Legion troopers, I kind of lost interest. I told you, I'm an Imperial Guard player, not a Space Marines one. I only picked up this post when I ssaw you can substitute Blood Angels with Cadian infantry, and even then I'm wondering how the damned fluff works out when you still can't use Armageddon Steel Legion troopers instead of Cadians or Space Marines. As I said, it ain't Armageddon without Armageddon Steel Legion, and I'm annoyed that the Imperial Guard regiments of their homeworlds are often left out in favor of the more recognizable Space Marines. Just look at the poor Prosperine Spireguard. If I want to play Blood Angels, I'll have them in the hive cities of Baal or something, not in Armageddon. Armageddon is Armageddon Steel Legion. No compromise, no surrender. For Lord Commissar Yarrick!
Speaking of which, for those of you who are interested in the stories of the Armageddon Steel Legion and Commissar Yarrick fighting against Orks, I've written a couple of book reviews on the Astra Militarum anthology as well as Yarrick: Pyres of Armageddon. Those tell the stories of Armageddon a lot better than the Space Marine ones.