A Walk in the Dark A look in to the mind of an RPG designer


25Aug/190

Atomic Age – Dice Mechanic

This is the FIRST article in my series on the design aspects of my new RPG in-the-making... Atomic Age!

In thinking about what Atomic Age is going to be, one has to start at the thing that is the basis for the entire system: what dice to use. There are so many different systems out there that it's not as easy a decision as one would think, and it all depends on what you want to get out of the system.

Expectations

So, first of all, let's define what we want to get out of the system:

  • Do not overcomplicate the math. I don't want a system that will involve adding eight different numbers before I can determine if the roll was successful or not. So, basically, I want something along the lines of [die roll]+[modifier] >= [target number] to determine success or failure.
  • Make it flexible, so that the die rolls can be improved or hindered in a variety of ways.
  • Make it easy for people to understand and relate to.

Let's get one thing out of the way: although I appreciate and acknowledge all the many d6-based systems out there, I want Atomic Age to be based on the d20. But there's more to it than that.

I also like to have a mechanism by which, the more of an advantage you have, the more dice you roll. Mainly because it makes the advantage feel more tangible, and players do like rolling dice after all. But I also don't want a system where the players end up dumping a Shadowrun-sized vat full of d6s on the table and then have to calculate up all the dice.

Probability Analysis

For all the probability analysis that will follow, I'm going to use AnyDice to generate charts and probability math.

As for what we will use as a basis for the math, let's assume that you're making an attack with a base +5 attack bonus against an AC of 15.

D&D 5E

The concept of "roll two d20s and pick the better one" is not a new thing... it's been around for some time, and there are several systems that use it. Dungeons and Dragons 5th Edition gave the mechanic a name: "advantage" and "disadvantage".

In past editions, figuring out a roll involved adding a lot of numbers. Attribute bonus, proficiency bonus, attack modifiers, target DC modifiers, etc... The notion of "advantage" was reflected by simply adding more modifiers to your roll so that your end result is more likely to be higher than the target DC.

The 5th Edition mechanic of advantage/disadvantage simplified all that. Your modifiers generally don't ever change due to circumstances (there are some exceptions, like adjusting AC due to cover, but still), and if you're in an advantageous position, rather than add more numbers to your roll, you simply roll two dice. It's easy to resolve!

It has one drawback, however: "roll two dice" is the ONLY thing you can do. "Rules As Written" you can't roll three or four if your situation is even more advantageous. For example, consider these possible attack rolls:

  1. Attacking a target that you are flanking with an ally.
  2. Attacking a target that is paralyzed.
  3. Attacking a target that is unconscious.

In all those situations, "advantage" is pretty much all you get, although attacks #2 and #3 should clearly be significantly more advantageous in terms of your ability to hit. Attack #3 has the added benefit of being an automatic crit or "coup de grace", but the chance of hitting is the same; it's an advantage attack roll against the target's AC.

"Rules as Written" there is no mega-advantage mechanic. There is nothing documented where you would roll three d20s. Except for spell modifiers (we'll talk about that later), it's always two and only two d20s.

Probability

So take our probability example... A standard die roll of 1d20+5>=15 has a 55% chance of succeeding.

With advantage, that probability rockets up to almost 80%...

...and with disadvantage it plummets to 30%.

Disadvantage is brutal in 5E; when rolling disadvantage, the probability of success plummets dramatically.

And, as mentioned above, that's it. If you have a superior advantage, it won't be more than 80%. It is what it is, pretty much always.

Shadow of the Demon Lord

Rather than use the "advantage" mechanic of 5E, I looked to another system for inspiration... Shadow of the Demon Lord.

SotDL uses a system where you still roll a d20 and add modifiers, but you can also add a "boon" or a "bane". A "boon" is adding a d6 to the d20 result, while a "bane" is subtracting a d6 from the d20 result, and they cancel each other out. If you have more than one boon or bane, you roll multiple dice and choose the higher result.

Personally, I like this mechanic for a variety of reasons...

  1. It physically acknowledges a superior advantage. If you have a high advantage, you'll be rolling a fistful of d6s.
  2. Even with a fistful of d6s, the probability does not increase linearly.
  3. It allows the boon/bane dice to be modified using external abilities.

Probability

So the base probability remains the same... 55% success.

... but, instead of advantage, we add a "boon" d6. That increases the probability to a little over 72%, which is comparable to the 80% of advantage.

...and let's say you have two boons. It increases slightly, to 77%.

On the other side of the card, one bane isn't as painful as disadvantage; 37% chance instead of disadvantage's sharp drop to 30%.

...and two banes is still at 32%.

I kinda like this... The probabilities work out the same, albeit it might be a little more swingy, and the advantage or disadvantage is both visual and tangible.

How would this work? Well, take D&D spells like Bless for example... it normally adds a d4 to rolls. Using this mechanic, Bless will simply add one boon. You flanking someone? Add a boon. The target paralyzed? Add a boon. Flanking a paralyzed target? That's two boons total... make him pay!

13th Age

One mechanic I liked from 13th Age is the notion of increasing or decreasing a die roll one or more "steps". For example, if the base die of your attack is a d6 and a feat allows you to increase it one step, the base die becomes a d8.

The way I see it, this can be worked in to this system a little easier. For example, there may be a spell or class ability that will allow you to make your first boon die a d8 instead of a d6, or turn your first bane die into a d4 instead of a d6.

Conclusion

So taking all that into consideration, here's my plan for Atomic Age:

Bonus and Penalty Dice

I don't want to call them "boon" or "bane" for obvious reasons, so for now let's call them "bonus" and "penalty" dice. Standard die is a d6, and it may increase or decrease steps depending on abilities.

"Bonus" and "penalty" dice cancel each other out, and if rolling multiple dice you choose the highest result in the pool.

Attack Rolls

[1d20]+[ability score]+[proficiency modifier*] vs target AC

(NOTE: I'm debating keeping the notion of "proficiency"; more on that at another time)

Skill/Ability Checks

Untrained: [1d20]+[ability score] vs Target DC.

Trained: [1d20]+[ability score]+[trained bonus (TBD)] vs Target DC.

Modifiers

Have some sort of advantage (like Bless, for example)? Add one or more "bonus" d6s.

Have some sort of hindrance (like Bane, for example)? Add one or more "penalty" d6s.

Stuff To Be Determined

What I need to figure out still is what defines the modifier. I'd like to avoid the notion of a proficiency bonus or the linear escalation of numbers (which was absurd in 4E). In the best of all possible worlds, I'd like an average DC to always be DC 15 regardless of whether you're level 1 or level 10, although it would be more likely you'll succeed the higher level you are.

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