The Aurora RPG Engine – Part 5

Baseline Values and an Aside on Probabilities

The base number of dice for a skilled entry-level professional is baselined at 5 dice.  With 5d and a target number of 15, the chance of success is roughly 75% (in actuality it is a bit over, but close enough).  As outlined in this previous blog post, this falls into the probability range that I find is most perfect, where a competent, trained, healthy individual beginning their professional career should succeed on almost-routine tasks (I say almost, as if it were perfectly routine there’d be no need to roll a test!) most times.  And even in those instances where they fail their roll, rarely will it be too disastrous of a setback (especially if the player chooses to succeed at cost, which is further explained below).

By adjusting the dice pool up and down in increments of ½ dice (a d3) allows for roughly this progression of success chances:

3d 10%
3½d 25%
4d 50% (actual is 45%)
4½d 66%
5d 75%
5½d 90%
6d 95%

In graph form, it looks like this (courtesy of

In play, it is easy to remember that 3½d to 5d forms the middle ground of success probabilities.  Anything less than 3½d is super unlikely to succeed, and anything over 5d is pretty much guaranteed to succeed.

Taken together, everything comes together nicely:

  • A nice round target number that is easy to remember.
  • An easily graspable number of dice that counts as “competent”.
  • Enough dice to let beginning characters succeed despite modifiers while also tempting the players with extra actions.
  • And an intuitively graspable notion of the chances of success.

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The Aurora RPG Engine – Part 4

Fixed Target Number and Difficulty as Dice Modifiers

There is one main disadvantage to most dice pool systems:  the time it takes to total the rolled dice.  This begins to get especially tedious around 7-8 dice and only increases thereafter, slowing the game and potentially killing its momentum.

A second disadvantage also arises if the system uses a series of increasing target numbers (for example, setting a target number of 10 for an Easy task, 15 for a Moderate task, 20 for Difficult task, etc).  Doing so undermines the intuitive feel of “Number of Dice = Chance of Success”.  If I have 8 dice in my hand versus 5 dice, I should feel as though my character is more capable; however, if the target number is also changing/increasing, then it’s tough to gauge whether those extra dice really are leading to an extra chance of success.  With two variables at play it creates a matrix of possibilities that hinders any automatic and visceral feel.

By setting the target number at a fixed value of 15 and by adjusting both for the difficulty as well as accounting for all other modifiers by adjusting the number of dice rolled:

  • The number of dice rolled at one time is generally kept low.
  • Even when many dice are rolled, we only need to count enough dice to make 15, which can be easily done by adding together the highest dice until 15 is reached. (The remaining dice are used as a “Margin of Success”, to further explained below.)  This avoids needing to tally large numbers of dice.
  • And because the target number never changes, we easily grasp the chance of success by the number of dice in our hand. The visceral nature of the die pool is maintained.

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The Aurora RPG Engine – Part 3

Base Underpinnings and the Dice Pool

At its fundamental level, this is a d6 dice pool system, chosen for the numerous of advantages it brings to the table:

First and foremost is the pure visceral aspect of a dice pool.  By holding a number of dice in our hand we gain instant feedback of our strength in that moment.  We feel it.  As characters progress in skill and ability, it’s immediately apparent through the number of dice.  So too is the impact of adding or subtracting modifiers.  At each moment in the game we feel our character’s chance of success (or not).  Altogether it is much more personal than a faceless target number, and as such the experience of rolling heightens our emotional attachment.

Secondly, to summarize this earlier blog post, it allows for an elegant way of handling multiple actions by a character in a turn:  for each declared action above the first, subtract one die from every test made.  This allows for a sweet differentiation between experienced and inexperienced characters while also elegantly handing movement and incidental actions.  Within this system there are no fixed silos of (arbitrarily increasing) attacks per turn, no rigid number of actions/moves/bonus actions, or the like; instead it provides a unified and organic method that promotes options, interesting choices, and crazy excitement.

Thirdly, it allows for various sub-systems and abilities (such as martial arts, stunts, equipment traits, or other similar things) where removing dice can be used to “fuel” special maneuvers or attacks.

Lastly, because we’re dealing solely with dice, both the number of calculations as well as the values involved tend to remain low.  There’s no need to add, for example, +17 to a roll.  Starting with a base number of dice (likely to be less than 10), then adding or subtracting a few dice (likely to be less than 6 either way) for modifiers keeps things simple.  Even if our list of modifiers grows large, because we are dealing with actual dice it remains easy to calculate things by going through modifiers one by one and physically adding or removing dice from our hand until the final value is reached. Continue reading