My Megadungeon Conversion and Prep Sequence

So you've got a big megadungeon for D&D and you want to run it in GURPS.  The adventure only has GM maps available in black-and-white (or even old-school anti-1970s-photocopier blue-and-white), and you want player-friendly full-color detailed maps in your VTT.  How do you get there?

The first step is to read the whole adventure before making the final decision to run it.  You don't want to start running the first level of the adventure because it started well, without knowing that level seven isn't finished or sucks and then start changing everything at the last minute once you realize you're in a corner.  That way lies Game of Thrones Season 7.  Converting a good adventure is much easier than finishing an unfinished adventure or fully rewriting a bad adventure, so make sure you like the whole adventure (or at least 95% of it with a concrete plan for how to fix the last 5%) before you start running it.  It's well worth a bit of reading to avoid starting something you won't want to finish.

The next part is going back to the start and figuring out the basic challenges at the start of the adventure.  What is the entry point, or in the case of a megadungeon with multiple entrances, what are the most likely initial entry points?  How hard are the challenges there?  Are there any likely first-session TPKs?  If so are you okay with that, or do you want to insert some hard or soft barriers to prevent them, so you don't lose your players before they get invested?  Hard barriers would be something like an unpickable locked door between the PCs and the Totally Out of Depth Boss Monster, where the key is a couple of levels down guarded by a Less Unfair Boss Monster.  Soft barriers would be direct warnings or fuzzier rumors that a certain area is really dangerous; PCs can still go there and get killed if they want, but they can't are slightly less likely to blame the GM.

Once you know how hard the start of the adventure is, you can figure out about how many PCs you want and their rough starting power level.  You need to know this before you start recruiting players if you don't already have them; some spoiled power gamers will want to skip any game where they don't get to start as a demigod, while some old-school hardcore types will dismiss any game where their PCs start already knowing how to walk as too easy for them.  The point isn't to judge anyone's preferred style of play; it's to honestly communicate so that you get players who want to play the same kind of game you want to run, so everyone is more likely to have fun and stick around.  You also kind of need to know this before you start converting the first level of the adventure, so you know where to set challenges.  For example, if you have a D&D module where the adversaries on level 1 are mostly small groups of orcs with 1 hit die, designed to be a reasonable challenge for level 1 D&D PCs, then if you're starting with 125-point GURPS characters, maybe those goblins should have weapon skills in the range of 12, damage low enough that they're unlikely to down a well-armored warrior with a single hit but high enough that the squishy mage feels vulnerable, damage resistance somewhere around 2, maybe some sub-optimal weapon choices like unbalanced knobbed clubs and no shields instead of harder ones like broadsword and shield, and use mook rules so they go down or run away after one solid hit rather than fighting forever like horror movie boss monsters.

Then I think you want to fully convert enough of the dungeon for the first few sessions.  You don't need to convert a whole 15-level megadungeon before you start running it; if you or your players don't love it and you quit a month in, you don't want to have spent months of prep time.  But you want to be far enough ahead of your players that there's no significant chance of them hitting a totally unprepped area.  I think doing 2 or 3 levels is enough to start, depending on how big the levels are, how many accessible entrances there are, how long your play sessions are, and (if you know your players) how likely your players are to crash dive versus make sure the current level is pretty clear before going deeper.

What comes first, the maps or the room descriptions?  Up to you.  You already have both the maps and room descriptions from the original adventure, so you have a place to stand either way.  My advice would be to do one at a time though, as switching modes between editing maps and editing text has a cost.

My choice is to first look for nice VTT maps that other GMs have made available.  If you run a very popular old adventure like Keep on the Borderlands or Ravenloft, they will definitely be out there.  (But it will be harder to find players who aren't already familiar with the adventure.)  If you run something newer or more obscure, they may or may not be.  Also, it's common for GMs to start running a megadungeon and not finish it, so you'll find a lot more level 1 maps than level 15 maps.  Once you find or make map images, keep in mind that you might also need to make time for manually drawing line-of-sight-blocking walls, and adding dungeon dressing, and testing your maps with an NPC token to verify that they really can fit through the narrow passages and the secret doors aren't too obvious.

As far as converting the room descriptions, I see three major phases.  I don't need to convert plain text, as text is universal across game systems.  But I need to convert monsters, and treasure, and ability/skill checks.  I find the skill checks the easiest to convert, so I do those first.  Need to roll 4d6 under DEX to make this climb?  I'll call that a straight Climbing roll (which defaults to DX-5) in my GURPS conversion.  Lifting the lid off this sarcophagus needs a Bend Bars check?  I'll call that a ST-6 check and say there's enough room for up to 2 PCs to participate.  Maybe give a bonus if they use a lever or something.  I don't need to anticipate everything players might do, just give a starting point for the challenge level, so when they do something unexpected my level of needed in-game improv is something like "I'll call that a +3 bonus" rather than having to rethink the whole framework.

Monster conversions basically fall into two buckets: easy and hard.  The easy ones are the ones that someone has already converted.  If there's a common monster like a goblin, there are probably already 7 GURPS versions of it, so I pick one.  I check my piles of official GURPS books, Gaming Ballistic's Nordlond bestiary, the GURPS wiki, Enraged Eggplant's blog, do a web search for "GURPS monstername" in case someone else has it on their blog or it shows up in a forum post, etc.  I might tweak it a bit, then I'm done.  If it's a monster that I can't find an existing conversion for, because it's obscure or sometimes only found in this one adventure, I do it myself, and that's 30 minutes of my life gone.  (More if it's a really complicated monster.)

Treasure conversions also come in two kinds, easy and hard.  For really basic stuff like coins I make up a formula and stick to it.  First I read enough of the adventure to see how generous or stingy it is with treasure, then I make something up like 1 gold piece in the adventure means $1 or $5 or $0.1 or whatever in GURPS dollars, then I can translate the total value back into flavorful fantasy coins and gems.  One caveat here is that it's easier to add more treasure later than to take away treasure you have given, so err on the side of less generous to start and then become more generous if it becomes clear you need to.  Stuff like gems and jewelry is about as easy as coins.  The hard stuff is magic items.  Some of the simpler ones are formulaic, like you might say that a D&D +1 longsword becomes a fine, balanced GURPS broadsword.  But if you have to convert something like a Rod of Lordly Might that has like 7 different powers, it can be tedious.  Note that if something is too annoying, you can just substitute something else.  Your players won't know, and as long as the substitute item is something they like, they won't care.  D&D modules tend to be full of scrolls and spellbooks, which are important in a Vancian system, but not so much in a point-based mana point system, unless they contain secret spells that are otherwise hard to learn.

The other thing to think about is the starting town or towns.  Is town a safe zone or part of the adventure?  What goods and services are available there?  What is in ample supply, what requires an availability roll, and what requires a trip somewhere else?  It makes sense to detail town a bit before starting, as your players might want to take the location in mind before making their PCs.  ("The only temple in town is to Mitra?  What a great coincidence that my PC is a devout, consistently-tithing follower of Mitra!")  I don't think a mostly-safe town typically requires a lot of game mechanic conversions, though I personally convert things like "5% chance of a minor magic item appearing in the market" to "minor magic item appears in the market on 4-" just to use fewer kinds of dice and simplify my VTT macros.

My final conversion tip is the TODO note.  If there's something that's really annoying to convert and you just don't feel like it today, leave a TODO note in your conversion and move on to the next thing.  When you finish your first pass at converting the level, search for TODO and maybe go back and do any that seem important, and leave the others for later.  The point is that you don't have to do everything in order, but you do want to remind yourself if you skipped something so it doesn't surprise you in play.

Once you've got enough of the adventure converted to start playing it, you have to make the decision to actually start.  It's tempting to say "one more level to be safe", and if you enjoy prepping more than playing that's fine, but if you don't it could be painful to prep more than you end up using, so try to find that balance between fear of your players overtaking your prep and fear leaving prep unused.

Once you actually announce the game and start recruiting players, keep in mind that much of the time you previously spent prepping the dungeon will instead go to talking to prospective players, introducing new players to each other, helping players build their PCs, etc.  I find this stage the most difficult part of the process of running a campaign.  When you're just prepping you're just prepping and it's just you and nobody else cares so your decisions have no real impact (yet), and once you get into the swing of a campaign it gains its own momentum and if it's going well the players help you keep it going, but the first time you post in some Looking for Players forum looking for the right 5 players and not being sure if you'll get zero or 50 is a bit tough.

The first few weeks of a new campaign are pretty busy dealing with new players and new PCs and (maybe for some players) new rules, so you might not have much time for dungeon prep.  This is why it's important to prep ahead enough to give yourself some runway.  At some point things calm down: the PCs (or maybe just some of them) survive their first few delves, the players (or maybe just some of them) like the game enough to keep playing, the GM handles things well enough to keep running it, the new players get to know each other and start helping each other so the GM doesn't have to do everything, and you start a regular loop.  For me, running a weekly game, it looks like this:

• Friday: game night.  Send reminders ("Game tonight at 8, get me your updated character sheets by 7 if you want the updates in effect for this session"), do last-minute review of the part of the adventure they're most likely to visit, actually run the game.
• Saturday: Post-game writing.  Write a recap, sum up all the loot found, award XP.  See if any players have immediate responses like "we need to find a wizard we can pay to identify that sword" or "we're definitely going after the lizardmen next week" that can give clues on what to prep next, and add notes to the todo list.
• Sunday: Short-term prep.  Go back through the last session's log and mark dead monsters as dead and taken treasure as gone in your conversion notes; you remember well enough now that you don't think you need to, but you probably won't remember so well when your PCs come back to this level in 6 months.  Resolve any town stuff that can be resolved out-of-game.  Make any short-term changes in the areas of the dungeon affected by the PCs' recent actions.  (Do the ogres put new guards in the upper guard post to replace the ones that got ganked?  Do the oozes to the west take advantage of the portcullis the PCs left open to add themselves to the random encounter table for the east?  Do we need a couple of new rumors to replace some that got used?)  
• Monday: Announce the next session and ask the players who will be available for it, to see if you have a quorum and help you and your players plan ahead.  (Alice: "Bob's not going to be here Friday so we won't have Gronkar the Tanky, so maybe this week is a bad week to do a frontal assault on the gnolls."  Clarence: "Maybe we should consider a scouting mission on the swamp level instead."  GM: "I haven't looked at the swamp level in 6 months but I'll add that to the todo list.")  
• Tuesday-Thursday: Long-term prep.  Once I'm confident that whatever we probably need for this week's session is ready, I can go back to working on the unfinished level 3 levels below the deepest the PCs have been.  For a simpler dungeon you might just go top to bottom.  For a more complicated and less linear dungeon you might be thinking in terms of what unprepped area they're most likely to discover next, rather than just prepping in order.  (You won't always guess right, which is fine.  Make your best guess, and keep prepping something almost every week, and eventually you'll get to all of it.)

In theory, eventually you'll have the whole dungeon converted and prepped, and you can drop the long-term prep part of the weekly loop and only make little tweaks in response to your players' actions, until they finish the campaign.  Now you're on GM easy street, basically running an adventure on rails, like people who run adventures for the systems they were actually designed for without changing anything.  If that gets boring, you can always start converting another megadungeon...