Wednesday, 28 March 2018

Blood & Bile: Behind the Design

A couple of weeks ago I released my first PWYW product on DriveThruRPG, a minimalist RPG called Blood & Bile. I've had a few private conversions about the underlying mechanics since then, so I thought it might be worth writing a short blog post on the subject.

System Overview

Each player has five d6s, color-coded to indicate which trait they represent (blue for brawn, green for guile, red for reflexes). The PCs are vampires, so the overall dice pool also indicates how much blood they have remaining.

When you make a trait check, you choose and roll 1-3 of your dice; if the highest die equals or exceeds the difficulty, you succeed. Any dice that color-match the trait can be rerolled once each, and if you invoke an appropriate "asset" you add a +1 bonus to one die.

If you fail a "dangerous" trait check, such as combat, discard the lowest rolling die (representing your body burning blood to regenerate injuries). Furthermore, if you roll a double you also discard one of the dice (representing burning blood for supernatural effort), while on a triple you discard two dice. Discarded dice can be recovered through feeding.

Crunching the Numbers

A character's chance of success depends on the difficulty of the trait check, and the number of dice they roll, as follows:

Although you can only roll a maximum of three dice for a trait check, you are allowed to reroll dice that match the trait, which means those dice each count as two for the purposes of calculating the chance of success. For example, if you roll two red dice and one blue die for a reflexes check, that would be the equivalent of rolling five dice, because the two red dice could be rerolled on a failure.

This means that (for example) rolling one red die for a reflexes check has the same chance of success as rolling two green or blue dice. However the red die has the advantage of not being able to roll a double on its own, while the blue or green dice would have the possibility of achieving two success when making an extended challenge.

Doubles and Triples

There's normally a 16.67% chance of rolling a double with two dice, while rolling three dice has a 41.67% chance of a double and a 2.78% chance of a triple. This might seem very high, however the risk is greatly reduced when rolling dice that match the trait check. For example if you roll two dice for a reflexes check, and one of those dice is red, there's only a 2.78% chance of rolling a double—and if both dice are red, there's only a 0.46% chance of rolling a double.

Although characters don't gain additional dice as they advance, they do earn assets. By invoking an asset to add +1 to one die, you can reduce the chance of rolling a double on two dice (or a triple on three dice) to 0%, and significantly reduce the chance of rolling a double with three dice—particularly if some of the dice also match the trait.


Assets are a simple mechanic that can represent skills, knowledge, equipment, allies, supernatural abilities, etc. It's best not to choose something too narrowly focused or overly specialized, otherwise it can be difficult to incorporate the asset into the story.

An asset can be invoked to add a +1 bonus to one die, which is the equivalent of lowering the difficulty by 1 (at least for the standard trait checks; extended trait checks track multiple successes). This bonus can also be used to break a double, or turn a triple into a double, which makes "supernatural effort" a far less risky prospect, as mentioned earlier.

Although they can only be invoked once per session each, your assets are always invoked after rolling, which means you only use them in situations where they make a difference. An asset never needs to be "wasted" on a roll where it has no impact on the outcome.


The game mechanics in Blood & Bile are obviously (intentionally) fairly simple, but I still wanted players to be able to make tactical decisions when rolling the dice, and I think I succeeded in that aim. I initially had doubts about the "asset" rules, but they've grown on me—they are mechanically extremely simple, but they provide the player with some useful options, tie in nicely with the narrative focus of the game, and can cover a wide range of different character abilities.