I've never done any kind of conversion/adaptation.
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it seems to me that, for myself, converting/adapting would be more difficult than just original creation based on existing play + PC build + my own imaginings
Over my RPGing life I've done a lot of conversion:
* I've converted a lot of D&D material (especially AD&D, but also 3E) to Rolemaster. This is mostly technical stuff around stats and expert knowledge of mechanical systems for the games in question.
* I've converted other bits and pieces to RM also: some Palladium, some Bushido, probably others I'm forgetting. The same is true of this - it's basically an exercise in technical expertise.
* I've converted both Moldvay Basic material (ie bits of B2) and 3E material (ie the module Maiden Voyage) to Burning Wheel. This is not just technical, although that is relevant in (say) converting a monster or NPC. It has a lot of what I described in my post just upthread, about thinking how a situation that is either presented statically (classic D&D) or a bit railroad-y (3E) can be adapted to BW's player-driven system. In the case of Maiden Voyage, for instance, this meant combining what the module presents as two appearances of the ghost ship into one single crescendo.
* The most recent Torchbearer adventure I ran - the Shadow Caves beneath Megloss's house - included elements I adapted from the old MERP (so, more-or-less, RM) supplement Southern Mirkwood, which includes Dol Guldur. This was mostly technical, because I wasn't really trying to maintain any of the overall structure/framing of the material, but already knew how I wanted to incorporate it into my adventure.
Of course, Torchbearer already has ample conversions built in - Bugbears and Gnolls and creeping oozes, for instance, which figure in the moathouse; and stirges (relabelled stryxes) which I included in my first self-authored Torchbearer adventure.
As to
why convert: mostly because I like a map, or a situation, or a character. Or some combination of these (eg ICE's Dol Guldur had a cool "seeing throne", which I adapted for the Shadow Caves and which did indeed prove to be cool). This can also be linked to nostalgia or shared experience: in Burning Wheel, most of my players recognised the Keep on the Borderlands when I described their PCs' arrival at it.
It can be harder than creating from scratch, or easier. In the case of the moathouse, I don't think I would have been able to write such a lengthy and in some ways complex dungeon myself, without inspiration or model, in the same amount of time as the conversion took me.
There are four components to TB Adventure design as I see it. These aren't in order of importance (they're all equally important), but rather, the order in which I think on/derive these things:
1) Proximity to Town and Journey Attrition (Toll if you're using the TB2 Loremaster Manual Toll procedures rather than TB1)
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2) Expedition/Delve Geography and Challenge Types - Sorting out what is there, their spatial relationships, and ensuring a nice mix of challenge types
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3) Problem Areas/Obstacles, Rest/Camp Areas, & Twists - The number of these, the list of Twists, and their difficulty/danger is encoded in Adventure Design.
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4) Dramatic Needs of the Ecosystem & Loot - <snippage> The place itself is "alive" (including its denizens) and each constituent part wants something (which necessarily almost always includes to crush the PCs' underfoot!).
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Of the four components listed above, what was the most difficult aspect during your considerations/conversion process?
In my conversion process, (1) and (4) were closely related, and I think (or at least hope) that the conversion reveals that.
The conversion locates the moathouse geographically in relation to the (ruined) Temple and to a (unspecified) Remote Village. Of course, and as per the OP of this post, the Temple in question is the Forgotten Temple Complex where the Dwarf PC Golin in my game is from. And the way Lareth is described establishes his oppositional relationship to this place, where Golin's mentor resides, and also to the Wizard's Tower which is where one of the other PCs, Fea-bella, is from and which for the moment is the PCs' main "home base" (because Fea-bella can stay with her mum, and Golin with his alchemist friend who lives there). A third PC, the skald Korvin, has as his enemy Nob H the Bandit Lord, and so the moathouse with its brigands and raiding is also naturally connected in that way too.
As we haven't yet had any play in the vicinity of the Forgotten Temple Complex, the details of geography (and things like Toll for travel to the moathouse) haven't been worked out yet, but I have pretty faithfully preserved the narration of the original module and would use that to place it on a map - luckily the relevant part of the map has an evil swamp (the Troll Fens) and so that shouldn't be hard to do if/when the time comes.
Doing these (1) and (4) bits wasn't that hard - I had them in mind in being motivated to do the conversion, and I think I have reasonably good instincts for doing the sort of work that (4) asks of a GM.
(3) wasn't too hard in intellectual terms, because I didn't have to think about this too much - it follows from the module that I'm adapting, including its map and key and its wandering monster tables. But it is probably the bulk of actual writing-up time. In converting monsters, I tried to first locate them within the D&D milieu in relation to a creature that already exists in TB (Dire Wolves and War Wasps carried a bit of weight here!) and then write them up with an appropriate Nature and Might, adapting special abilities and special weapons from other, appropriately similar, creatures already in the TB materials. I don't think my creature write-ups break any sort of new ground!
For obstacles, I took the approach of working out the odds in the D&D version; then next converting this to TB assuming a two to four dice pool as seemed appropriate to the likely context, and setting a difficulty on that basis; then finally checking this obstacle against my list of obstacles taken from the rulebooks and Cartographer's Companion to check that my fiction and my obstacles are consistent, and tweaking if necessary. (But I didn't have to do much of this tweaking.)
You'll have seen some of my suggested twists in the write-up, and mostly these follow pretty organically from the D&D write-up. (This is why I think of TB as doing classic D&D better than classic D&D does!) Rest/camp areas also emerged pretty naturally, and I don't think this is a coincidence given what I'm converting. The thing I'm least happy with is the preserved rations in the rubble-concealed storerooms in the dungeon entry. For Gygax maybe these serve a need for verisimilitude? But in TB I think it is a possible break-point, and I need to think about tweaking that before actually running this. The Shadow Caves has a source of potable water in it, which the players have exploited for camping, and so I'll have a bit more of that experience to draw on to help me think about how to do the provisions before I need to make a final decision. (I don't think the essentially endless supply of polearms is apt to be an issue in the same way.)
(2) is something that I had an eye on, and you'll see some notes that reflect this especially around the ghouls, the Gnolls and Lareth. It is certainly not a fighting-free scenario, but I think there is scope for a wide variety of approaches. And these will interact with camp danger levels in good ways, I think (eg driving off the lizard compared to killing it; or the more complex range of possibilities with the brigands, Gnolls, Bugbears etc).
What is missing from the scenario, and relevant to my PCs, is a chance to use Scholar in the adventure itself. (Lore Master feeds into Trickery and I think there is ample opportunity for that.) I think this is a legacy of it being a D&D adventure. But I think the chance to use Scholar and/or Circles to gather information in the Forgotten Temple Complex probably opens up enough space in this respect that I don't feel too worried about it.
Hopefully the above answers your questions to some extent!