GS: TTRPG - Core Dice System

I've been calling it the "Harmony Dice System" privately to myself. Which is distinctly less private as of this posting. It was inspired by the idea of finding balance between two chaotic elements. The idea being that this core system is flexible and compact enough to be used in pretty much any game, and with pretty much any two dice. It is, I believe, my own invention, or at least it's something I've never heard of before and that I think came entirely from my own mind. Were I ever to publish it, however, I think I'd do it publicly under an open license, cuz I want people to use it, to play around with it and try it out. I've been messing around with this system a lot on my own, and it feels pretty solid to me, so this is the system by which most of the rules in the GS: TTRPG revolve around.

How this works:

When the GM determines a player character is faced with a sufficient challenge, one with reasonable consequences should they fail, they will ask the player to roll for Threat. Threat is a numerical value determining the overall strength of the challenge in front of them.

The player then rolls two of the same polyhedral dice, in GS: TTRPG we're using d20s, and takes the difference between the rolled values (IE. subtract the lower value from the higher one).

The leftover value is the Threat of your challenge, and the player must then choose their approach and pay the value down to zero using their character's Attributes. If the player rolls doubles on the dice, then their Threat is already zero. This is counted as a critical success.

If you can pay down the Threat to zero, that's a simple success. If there's any Threat remaining once the player has exhausted all of their options to resolve it, however, the GM resolves any remaining Threat appropriately. 1-2 might be considered a partial success, 3-5 might be a simple failure, 6+ might be a complicated failure, with 9+ being a catastrophic failure.

An Example:

The PC is standing at the edge of a chasm, and they ask the GM if they can try to jump over it. "You can certainly try," comes the usual response.

The player rolls 2d20. They roll a 7 and a 12. 12 minus 7 is 5, so they're faced with 5 Threat.

The player then checks their character sheet. To simply jump across the chasm, it makes the most sense for them to use their Grace attribute, which, still pretty early in the game, is only 3. They subtract their 3 from the 5 and are left with 2 Threat. (There's other options available to the player but we'll explore those more later, for now they decide to risk the 2.) At this point, the GM decides to resolve the remaining 2 Threat as a partial success. The character jumps, but doesn't quite clear the gap and nearly slips back into the chasm. They are able to grab onto the ledge before slipping into the abyss, however. They are left in a precarious position, and can now try and pull themselves up, or wait for someone else in their party to help them.

Why is this any different from rolling 1d20 + skill versus a DV?

Well for one thing, it takes the chaotic element, the dice, out of the pool representing the player character's skills and moves it to the other side of the equation: the challenge. In most other systems, it'd be the GM who has to decide this value for everything. Here we leave it to the dice. This makes sense to me in the post-apocalypse, as any given door you're trying to kick in will have a variable level of rust or other blockage, making the environment the element of the game that's less predictable, while the characters' capabilities remain consistent.

This system also has interesting statistical benefits. Values are weighted more towards the lower end, with a Threat of 1 being far more common than a Threat of 19. The average value of Threat on 2d20 is about a 7, so extreme outcomes, while still quite possible, are less common overall than simply rolling a 1. This also means there's diminishing returns on attribute point increases. Increasing one attribute from 1 to 2 will increases your chances of success in that area by a much greater percentage than increasing one from 10 to 11. Players will see more benefit in spreading their point gains out in different fields, rather than doubling down on trying to achieve guaranteed success in just one area. This also means there's a soft cap for level increases. Eventually you'll reach a point where you've guaranteed success except in extraneous circumstances. I won't stop you from continuing to increase that attribute if you want to, but know that you're doing so at the detriment of the others.

Scaling Difficulty

Now what if the GM wants to fine-tune the challenges to allow for a little more excitement? Maybe they're dealing with higher level characters or they want to present the players with tougher challenges?

You could add modifiers to the rolled Threat, but when I do the math, what seems to happen is adding a +1 or a +2 Threat to the dice is statistically the same as subtracting a 1 or a 2 from your player's Attributes. It also gets weird with criticals, because how do you treat it when the player rolls doubles, but the GM has added +2 Threat to the roll? Is that still a critical success, negating the modifier? Or does the additional Threat need resolving still?

So I think the more interesting option for higher level characters is to scale the dice up instead. By using d22s, d24s, or even d30s, you're effectively pushing the average Threat higher, while increasing the volatility of the challenge, and reducing the player's chance to crit all at once. This also works in the other direction, with d18s, d16s, d14s, lowering the average Threat while also increasing the player's chance to crit. I've defaulted to d20s because these niche tiered dice can be more expensive or harder to acquire, but free digital dice rollers exist for these cases. The point is that if you WANT to scale difficulty, I suggest doing it with tiered dice, rather than directly modifying the Threat.

So I hope you found this interesting! I'm kind of enamored with this system and I'd love to hear your thoughts on it. Next time I want to go over my rudimentary rough character sheet, and explain Attributes and Vitals.