So, me and some others made some alternative mass combat rules for a Black Crusade game we played, which should also be compatible with other FFG 40k games. And having already made it, we thought hey, why not release it into the wild? So here it is . Why not use the existing Tome of Blood or Tome of Decay systems? Well, neither of those systems had the emphasis on commanders (and how assassinating them could turn the tide of battle) that we wanted. Plus, both of them differentiate little between the different combat doctrines of various forces. The Tome of Blood doesn't really scale up to sector-wide or even planet-wide conflicts, and the Tome of Decay is very much geared towards fighting the Imperium, with reinforcement rules that don't make much sense for extremely aggressive enemies like Tyranids or Orks. This does mean that these house rules are specialized into a game reminiscent of Assassin's Creed and that the Tome of Blood and Tome of Decay rules are generally superior if you're doing the default "lay waste to slow-reacting Imperium forces" thing that most black crusades do.

One other benefit of this system is that it's faction-agnostic, though, which means you can use it to run a Dark Heresy/Rogue Trader/Deathwatch/Only War campaign where you fight for the Imperium, and if you house rule up how to actually play as Orks or Eldar or Tau or whatever, you could use these rules for them, too. Technically not the Tau, because we haven't added that faction yet, but it's coming.