5 Tips for Converting Rules to a Different Game System Mid-Campaign
On Tuesday, I got an interesting mechanics question from Roleplaying Tips GM Ajax:
Thanks for the heads up on Blackbirds RPG. I have been slowly getting Zweihander supplements, either to run with the core rules or hack for use with other Fantasy RPGs.
I’m curious, have you ever written an article on using one RPG (taking concepts, classes, or rules) and adding them to another RPG with totally different rules mechanics?
I’d be very interested in your take on any particular game that you pulled from for a d20 fantasy RPG.
Hey Ajax. Interesting question! Short answer is no, not exactly.
For my homebrew campaign, Basilica, I’m currently translating Old School Essentials (D&D Red Book) to the WOIN RPG.
This isn’t exactly apples-to-apples with what you’re looking for, though. But some of the steps I’ve taken might be of interest, usable for translating X game to Y system.
5 Tips for Converting Rules to a Different Game System Mid-Campaign
1. Keep It Simple, Johnn
I kept the translation mid-campaign from OSE RPG to WOIN RPG pretty basic so we could get playing faster. Let’s call it the Bill O’Reilly approach to campaign migration (NSFW). 🙂
I’d rather get to playtesting stuff and finding the problems right away, at low levels, before much has been converted, and while everyone is still bracing for bumps.
For example, the first thing I converted was the dogman creature I brewed for Duskfall. That conversion taught me a lot about specific differences between systems.
Then, the party encountered dogmen immediately next session so I could playtest the conversion and get a better idea of what to do with the other monsters.
In addition, I asked players to translate their characters from OSE to WOIN. That helped them learn the new game, it was one less thing I needed to do, and it simplified things for me.
2. What’s My Cover Story?
I needed a way to explain how in-game reality suddenly changed. No matter how smooth a mechanical conversion goes, things will be noticeably different in gameplay.
For example, the OSE combat turn is a lot different than WOIN’s. Basic D&D has four phases to combat:
- Declare Spells & Movement
- Determine Surprise & Initiative
- Winning side does Morale, Movement, Missile Attacks, Spells, and then Melee Attacks
- Other side acts
Whereas WOIN’s combat turn goes like this:
- Determine Ambush & Initiative
- Highest Initiative goes first and can perform Actions in any order
- Next combatant acts
OSE is team-based and WOIN is individual-based.
That’s a huge change in reality. And I needed the campaign narrative to explain this to preserve the group’s suspension of disbelief.
Basilica is my homebrew homage to Temple of Elemental Evil.
Fortune favored me here because I had already described an ominous red cloud geosynched to the area, sitting right above the evil temple, spreading slowly in the sky. I scooped this idea from a Dennis McKiernan book, and it was going to function as a meta clock for a prophecy.
However, I had not introduced the prophecy into play yet, so I scooped the crimson anomaly for use as the Impending Doom of a game system change reality shift. “Blame the cloud.”
If the red cloud hadn’t been an option, though, I would still have quested for a Cover Story that would explain why the characters’ reality has changed.
3. Find My Key Numbers to Convert
I noodled on what mechanical elements I wanted to feel the same. Everything else I didn’t worry about and explained differences away via my Cover Story.
Monsters
I didn’t worry about porting all my OSE stat blocks to WOIN. In most cases, I translated a typical OSE stat block to WOIN for a tier or type, then I reskinned that stat block within WOIN to save time.
For example, the dogman monster translation gave me base stats for a low-tier humanoid critter. When I created kobolds and goblins, I reskinned the new WOIN dogman stats instead of converting from OSE.
Melee
I also looked at how many times it would take a PC to hit a foe before the foe was defeated, and vice versa. I wanted that same number, roughly, ported over.
Turns out, the characters were two-hit sponges in OSE, and WOIN offers more granular durability than that, so I just adopted WOIN’s economy there.
However, it was important to know how many hits OSE PCs were taking before getting KO’d so I could tell when I was veering away from the previous average and difficulty levels in WOIN.
Money
For the cost and value of things, WOIN and OSE both have gold pieces. So I decided the price of a longsword would be the key number for my currency converter.
If memory serves, the ratio was around 2:1. So I adjusted the cost of everything else accordingly, and then made spot changes where the numbers looked a bit off.
Magic
This posed the biggest difference between game systems. In WOIN, anything can have a MAGIC score and wield that to cast spells, use magic items, or exude natural magical effects.
Further, WOIN’s spells are dynamic. Build your spell, spend the points. OSE uses pre-fabricated spell lists and spells. Further, you need to make a dice roll to successfully cast in WOIN, but OSE spells are automatically hurled.
WOIN would make it easy to convert the casting system and spells of OSE. However, I like a dynamic magic system where characters and NPCs can make up spells on-the-fly if they have enough points left in their daily pool.
So little conversion for magic was needed.
Magic items are another story. Fortunately, we didn’t have many in the party yet, so I’ve been building items natively using WOIN mechanics. The few items that needed conversion were trinkets, and I used my Cover Story to explain why items suddenly had slightly different powers or usage criteria.
4. Make Detailed Session Logs
I take a lot of detailed session notes and mine these afterward for plotting, consequences, and world-building.
That’s how I knew my combat benchmarks for things like attacks and damage. I record attack and damage results as part of logging character actions. In Campaign Logger, this is very quick.
When we started playing my WOIN conversion, I kept up the logging so I could better triage and fine-tune translations.
For example, an encounter with a hag, giant toads, and some crocodiles nearly TPK’d the party. From my session notes, I spotted the problem. OSE has two defensive stats: Armor Class and Hit Points. But WOIN has three: Defense, Soak, and Health. The PCs could get past one or two defenses, but not all three. Perfect: lesson learned.
5. The Rest Is World Building
Well, not everything remaining is pure setting design. But with this core stuff figured out, most of my remaining prep was translating the different mechanics into setting changes and details.
For example, my WOIN dogmen can use their magic scores as part of rituals led by dogman shamans. Dogmen didn’t have magic rituals in OSE, so I needed to do some world-building to accommodate this fact.
However, I use my Spiral World Building method, so I only needed to accommodate a few base things to start, like why everyone potentially has a MAGIC score. Then I leverage gameplay and session notes to do continual, cumulative world updates.
Ajax, I hope these tips were of interest. I know they didn’t exactly cover taking a mechanic from one game system and porting it over into a d20 chassis. But a lot of the thinking is the same:
- Playtest fast
- Don’t boil the ocean – convert what you need for the next session
- Determine what gameplay you want to feel the same
- Create an in-game reason for why reality has changed
Cheers,
Johnn
roleplayingtips.com
https://discord.gg/6MxTRAqQ76
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