Obviously, the hard part is getting everything down on paper. I realize there is a lot of work to prepare this because you need to prepare several stat blocks for each zone and a roster for each faction (and some unaligned rosters). However, realistically, you do not decide to run an open-ended sandbox game without knowing you need to do a lot of prep.
However, once you’ve done that, Slaughterhouse minimizes the work you need to do between sessions (or during the session) to adjust for the party’s actions. If the party wipes out a Lair, you simply remove that roster, abandon any of their outposts, and use the stat blocks and rosters to repopulate any areas that need to be changed. This is quick work unless you’re doing it for a lot of zones.
Using the population thresholds and triggers, you can actually give a lot of personality to different factions. Cowardly factions will abandon outposts at the first signs of trouble. Other factions will dig in. Faction behavior might change depending on other events (such as Lord Vizier’s willingness to hold the Fortress after the Temple is claimed). You can build a lot of versatility and life through a very simple system.
I realize that Slaughterhouse is not for everyone. Some folks prefer to simply improvise, rewrite, and adjust in a less systematic way. And, I’ll be honest, I do too. But the massive undertaking of the super dungeon has required me to find a more systematic approach.
Slaughterhouse also allows large dungeons to be shared between DMs without having to stat up every encounter and development. If two DMs understand the system, all they need to share between them is the Rosters and the Stat Blocks along with story notes. There is no reason that a dungeon spanning ten experience levels couldn’t be shared in about fifty pages and a collection of maps.
And finally, one exciting possibility is to have several DMs run the same massive dungeon for different parties. If they are both working from the same collection of maps, rosters, and stat blocks, they can quickly correspond after each session. Party X and Party Y could both explore the same incarnation of Undermountain at different times, each one changing the shape of the dungeon for the other. The parties could even shuffle members between them.
What My Slaughterhouse Adventure Looks Like
As I said, my super dungeon has about 60 zones and about 30 factions, which is simply huge. But it is meant to fill a campaign. A smaller adventure will have far less than that. What does my super dungeon look like? As of right now, it isn’t quite finished, but these tools have let me work very fast to lay the groundwork. I don’t have to concentrate as much on placing individual encounters for every single zone because I have the tools to quickly shuffle things around. Presumably, if the party wandered into a place I hadn’t populated yet, the roster and the stat block would give me all of the tools I need to populate it (and repopulate it) in a very short period of time.
My super dungeon is currently contained in a big binder divided into two parts: zones and rosters. The rosters are simply stat blocks for all of the factions and numerous unaligned rosters for different zones and regions.
Each zone has a map, of course (and I also have several master maps to show how everything fits together). Each zone also has a single page for each of the possibilities I’ve written (e.g.: Lair of X, Outpost of Y, Outpost of Z, Unclaimed). That page includes a stat block for that state of the zone, one or two terrain features or hazards common to that zone that I can put into encounters, and any notes about skill challenges, traps, or other things that are a part of that zone (rather than part of a faction).
I’ve also put other tools into each zone for easy reference. I’ve got treasure listings, descriptive text (not a lot, I tend to improvise my flavor text) and so on. As far as tracking populations and populating encounters, I tend to write notes right onto the pages as I need to and I can easily remove pages that no longer apply (if a lair is abandoned, for instance, or a faction is defeated). But this is my way of organizing and that’s a highly personal thing.