The Mega-Dungeon is the Moby Dick of dungeons, an unseemly gobstopper of woe
Greyhawk Ruins... Castle Whiterock... Rappan Athuk... World's Largest Dungeon...
Their names are legendary and their numbers are NOT Legion. Mega-dungeons are a rare breed in the world of RPGs. Bridging the organizational gap between the "boxed campaign setting" and the "linked adventure series", the mega-dungeon is a beast unlike any other. Hordes of monsters, sprawling maps, excessively harsh pitfalls and often nearly incomprehensible plots... if any!
As challenging to create as they are to survive, most of them took years to organize, plan out and get into a publishable format. Few DMs have the temerity to introduce them to their players, and still fewer players have the stamina to endure them.
I remember the first time I perused a copy of Greyhawk Ruins. I was agog at how massive it was. Page upon page of encounter lists, bare-bones room descriptions, rosters, and maps, maps, maps! It was laughably huge, and like most people I was immediately interested in playing through it. I never was able to find someone willing to run it, though, and I wouldn't want to attempt it myself.
Many years later I came across Rappan Athuk, another >20 level dungeon crawl involving a brawl with Orcus as the module's final climax. Wow. World's Largest Dungeon followed. More wow. Then came Castle Whiterock, from my favorite D&D publisher of all, Goodman Games. Here was something I could sink my teeth into; a mega-dungeon, but cleverly broken up into pieces that could serve as set-pieces within an existing campaign.
I decided to run Castle Whiterock, but before I could finish off my current players' campaign the Pathfinder rules hit the market. Pathfinder is close enough to D&D 3.5 that one can make the switch with a minimum amount of pain and suffering, but it did interfere with my plans to run Castle Whiterock.
After consulting with Goodman Games and Paizo Publishing's legal department,I set forth to create an "ENWorld-style" adventure conversion for those wishing to run a Pathfinder version of CW without having to do the footwork themselves. Here are the lessons I have learned from this project so far.
1. Mega-dungeon's are BIG.
Converting a mega-dungeon over to a different game system is a big job. The smartest way to approach this is by breaking it into workable chunks of tasks. When I began work on Castle Whiterock, I made fair progress in the short term, bulldozing my way through the first level or two with little to slow me down. Then summer rolled around and I found myself spending less and less time working on the conversion. In order to drive forward, I now have to pick and choose what to work on first.
2. No conversion is clean.
Snags are a fact of life in the materials-conversion space. No matter how straightforward things might seem at the start, you will run into speed bumps - if not outright roadblocks. Even between relatively similar game systems there are idiosyncrasies that can significantly increase or decrease the challenge of a section, room or encounter. You must always consider the roll of the particular encounter and apply the new rules to it accordingly.
Consider, for instance, the humble Sorcerer. Castle Whiterock (written in D&D 3.5, remember) contains a number of sorcerer NPCs. While fairly lackluster in 3.5, sorcerers got a significant boost in Pathfinder with the addition of new abilities, bonus feats and so on. For the most part, it is a fairly simple matter to realign an NPC's abilities to suit changes to its profile brought on by using new abilities and/or rules. For example, Nemoura Shimmerscale -a nixie sorcerer found in Castle Whiterock - should probably have the Elemental (water) bloodline, although given her placement and purpose within the dungeon, a Fey bloodline could work too.
There are myriad possible examples of this just when moving from 3.5 to PFRPG, so you can extrapolate the difficulty of moving from something like a 1st edition adventure to Pathfinder. It takes a fairly skilled DM to manage that kind of transition efficiently.
3. Copy and Past is your friend.
Luckily for us, mega-dungeons tend to have one thing in common; there is a lot of repetition. Even if it is just the repetition of base monster types (generic troglodytes or gnolls, for example), the ability to re-use monster/race statlines is huge. This type of conversion is your low-hanging fruit.
I make free use of The Only Sheet. I've been a satisfied customer for years - even though I lost a fair amount of functionality in the transition from their 3.5 rules to their Pathfinder rules due to the loss of preset NPC races (not blaming!). It does most of the crunch for you and allows you to create your own classes, races and templates, which you can re-use whenever you need them. So I have a Castle Whiterock folder in my ultra-secret DM Lair which contains all the modified statlines for the unique and non-unique NPC monsters as I come up with them.
While converting old material to current material is not an insurmountable task, it is difficult, and a little preventative planning along with assistance from third party tools can be an enormous asset.