While I've been revisiting Velotha's Flock for a second edition, I've also been working on a new game, BIOREAKTOR.

The idea behind BIOREAKTOR is simple: I want it to be a very light game that feels full-featured.

Look at this cover, man! It's not the most brilliant thing I've ever seen, and it's probably not my favorite thing I've ever drawn for that matter, but I like to think that it's evocative.

The central mechanic I've decided on is stolen from inspired by Open Legend, a game I was actually a very minor contributor to, though I've got a very different take on it.

It revolves around a core d20, plus bonus dice to represent a character's skills and abilities.

This is more or less where the similarities end. I don't have exploding dice, and a lot of the mechanics are much simpler and streamlined.

Each roll has a Target, a basic number that a character needs to reach to succeed, and a Margin, which is a further threshold that gets additional successes and the like.

The d20 is added to by four possible additions based on characters, which are broadly lumped into two categories:


Domains give a die from a d4-d10, with the potential to boost up to a d12.

Each domain also comes with a variety of special abilities, both passive and purchased with Essence or Vita on activation.

Two examples of this:

Destruction, which gives its dice bonus to any destructive act (including most combat), as well as granting a passive boost to damage dealt in combat. An optional ability is the ability to reflect fire damage that you would have received to enemies.

Mystery, which gives its dice bonus to uncovering and concealing secrets, and lets people discover the domains and the awakened status of anyone they lay eyes upon.


Histories give that d4-d10 bonus, and can potentially boost up to a d12 like domains.

Histories are defined by the character's biography, which serves both as an explanation of who they are and as a source for the bonuses: they are drawn from individual words in the biography.

It's worth noting that histories are applied explicitly based on the spirit they manifest, not the literal meaning of the word. Someone who is described as "SHARP" doesn't get a bonus to use knives if it refers to their looks or fashion sense.

They are further split into three types. They all function identically, but only one history from each type can be added to a roll.


Affinities give a bonus based on a character's expertise or skill. They are expressed with verbs, adjectives, or adverbs (e.g. FIGHTS/FIGHT/FOUGHT, AGILE, or BRILLIANTLY), though they could also be used if the character is described using a noun (e.g. as a FIGHTER).


Bonds give a bonus based on a character's connection to the world around them. They predominantly modify proper nouns, but not exclusively (e.g. my brother THEODORE, his enemy THE INQUISITOR IN RED). You can also use this with something like a denonym or other modifier (e.g. ATHENIAN).


Tools, gear, and other objects and items that are not necessarily proper nouns are objects. These can include specific items, like a GUN or a LOCKPICK, or even something like a COMPANY or CONNECTIONS.

Bringing it together.

A FIGHTER with a HIDEOUT in ATHENS gains bonuses when fighting, when using that hideout, or when a connection to Athens is apparent. Because these are an affinity, an object, and a bond, the character could conceivably use all three simultaneously, like when fighting to protect his Athens hideout.


Vita/Essence is the health pool for the game; as the characters advance they sacrifice Vita for Essence by completing an Opus, an act of transformation that unlocks some of who they are at a fundamental level.

Essence and Vita are generally interchangeable, but Essence is more desirable because it recovers more quickly and can be used in place of Inspiration.

You can activate domain abilities with Vita or Essence, but it really hurts to use Vita while Essence is much cheaper.

One interesting counter-balance to this is that as you get more Essence the game encourages you to play your cards more radically. Essence recovers quickly, but if you're caught with it spent you're more likely to have everything come crashing down.


Inspiration is a tertiary resource, gained when you get good Margins on a roll or as a reward for good roleplaying.

Right now I know that it can be used to reroll a single die, taking the new result in place of the old one. I haven't come up with the full list of other things. It may also be used to regain Essence, but I'm going to want to try testing this out in a mature system before I settle on that.

The Opus and Progress

Characters work on their Opus as they develop their characters. Progress is measured in milestones; significant events in the character's life give Progress, and each Opus requires a particular amount of Progress.

When a character hits the Opus milestones, they upgrade their Domain, boosting its die and unlocking new domain abilities and convert Vita to Essence. Progress doesn't reset, and is also used to upgrade histories and purchase new ones.

The Math

So a character in BIOREAKTOR who is not good at something has a range of 1-20, with an EV of 10.5.

A character who is trying something that they have a domain interest in will, at character creation, have a range of 2-24, with an EV of 13.

A character who has a domain and a couple relevant histories will have a range of 4-32 at character creation, with an EV of 18.

A character in Opus 3, who has two relevant domain/history dice at d8, which is more or less what you'd expect, should be rolling a range of 3-36, with an EV of 19.5. This is also pretty what you'd expect from a character in Opus 2 with three relevant dice at a d6 (4-38, EV of 21).

The absolute potential max for a character in Opus 5 (the end-game, d20+4d12), is a range of 5-68, with an EV of 36.5.

You'll notice that while the upper range looks really great, that's going to be a very high-tier character.

I went with a Target+Margin system for two reasons:

  1. It's really simple to understand and eliminates the need for extra rolls and a lot of other moving parts.
  2. It lets us reward characters who are going off the mathematical scale.

Let's consider a Target and Margin combination of 10 and 4 (henceforth I'll us the notation Target/Margin, so in this case it would be 10/4).

For a starting character with no relevant strengths, this would succeed 55% of the time and get a +1 Margin 35% of the time, with a +2 Margin about 15% of the time.

Step up to a starting character with a relevant domain and history (or two relevant histories), and you get a success ~80% of the time, with a +1 Margin more often than a character without relevant strengths would even succeed.

Check out the Anydice calculation for yourself.

This is a really aggressive curve as we add larger dice (or more dice), but that's okay because I want characters to feel really powerful, and I also want to minimize the idea that characters need to add the absolute maximum to everything. Bonds in particular should be something that gives an extra boost.

The idea being that an realistically obtainable action should run from 10-25, with 25 representing something nearly impossible (the protagonists are psionic superhumans, after all, so them getting results of 25 is feasible even if they're relatively recently awakened), and anything above 25 representing something only possible in the realms of the supernatural.

Margins run from 4 (easy to achieve well) up to 8 (requires a master's touch to improve on).

I'm aware of the math required at the table, and it's somewhat heavy but not impossible:

STEP 1: Result >= Target (or FAIL, ending the steps)
STEP 2: Success = Result-Target
STEP 3: Success/Margin (discard remainder) = Degree

Now, mind you, this isn't exactly how I'd do it, but I don't think my mental schema is what will work for everyone. For something like 10/4, I'd just go 10=success, 14=success+1, 18=success+2, 10+(x*4)=success+x, but that's because I'm pretty good at considering stuff like that because it's very much in the game designer math you should know sphere.

Now, there are a couple things about this system that I really like:

First, I'm never going to have players roll dice more than once. You can reroll dice, but that's something you do after getting a result if you don't like the result, and it should be easy to intuit how the results differ (e.g. you might get +1 or -1 Margin, or go from failure to success, but probably not a whole bunch of different possible outcomes). Rerolling from failure to success, even with a margin, involves relatively known quantities.

Second, you spend more time, mechanically speaking, with successes than with failures. Psychologically, I like games where I feel successful and my failures are not dwelt on. Given the studies I've seen and my own experience as a designer, I think this is pretty reasonable to generalize to the larger gaming population. I think that BIOREAKTOR does that with this setup.

It's also worth noting that any significantly high margin (+3) typically will stop counting. Damage in combat is an exception to this, but everywhere else getting >3 just means you get a free Inspiration. You don't have to be totally beholden to accurate math on Margins, either.

Wrapping Up

BIOREAKTOR lets characters feel powerful over a relatively large scale of ranges and is built around having the most possible outcome from the shortest word counts and discrete rules.

I estimate it at about 20-30 pages (>10,000 words) all things considered, though my estimates have never been particularly accurate in the past so I'd take that with a grain of salt.


To listen to the audio version of this article click on the play image.

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