Subsystems – self-contained rulesets that didn’t interact with the rest of the rules – ran amok in 1st Edition and, by 4th edition, have mostly been removed. There is still a place for the subsystem in D&D. I like introducing one-shot mechanics to spice up a single encounter. Oxymoronically, I like to use a consistent structure for all my subsystems.

I use a mechanic I call “Saving Throw with Fumble and Crit”. I’ve tried to invent a cool acronym for it, but all I’ve come up with is either “F On Toast” (“Fumble Or Natural Twenty On A Saving Throw”) or “Stoat Ass” (“Saving Throw, One and Twenty are Super Special”). Let me know if you can think of an even more unacceptable acronym.

The basis of every “F On Toast/Stoat Ass” subsystem is a chart like this:

Make a d20 roll, plus any situational modifiers.
1 or less (or natural 1): Critical failure
2-9: Failure
10-19: Success
20+ (or natural 20): Critical success

The principles behind this chart are
a) that the 4e saving throw (essentially a coin flip that slightly favors the player) is a good generic mechanic, and
b) that “20 and 1 are Magic”.

I’ve used this subsystem template for my wilderness survival rules, mass combat rules, wandering monster rules, random treasure rules, and several other homegrown subsystems. It’s easy to explain to players, especially the second or third time the same structure is used. All you have to say is “Make a saving throw. Let me know if you crit or fumble.”

Rolling a 20

We’ve been conditioned to salivate when we roll a natural 20. It’s very satisfying to crit on an attack roll, but on many other rolls, all we end up with is a regular success and a mouth full of saliva.

Whenever a natural 20 is just another success, I feel like it’s a failure of the game system. Example: initiative rolls. I’ve frequently heard PCs complain when they crit on an initiative roll: “Why couldn’t I save that 20 for when it mattered?”

I was once DMing for three players who each rolled a natural 20 on the same initiative roll. It was an astonishing one-in-8000 occurrence that, sadly, has no by-the-book game effects at all. I ruled that the players were so well-prepared for the combat that their opponents immediately surrendered. The PCs got full XP for the win. We still reminisce about that encounter.

D&D design principle: Natural 20 is magic. Every d20 roll – skills and initiative rolls as well as attack rolls – should have a benefit for rolling a natural 20: something more than just a success.

Rolling a 1

Rolling a 1 is the second most exciting roll in D&D. I don’t know why it is, but it always gets groans and laughs. In every group I’ve ever played with, players narrate how badly they failed. “My axe gets caught in the floor!” “My Dungeoneering check was so bad, I don’t even know I’m in a dungeon!”

Players are hungry for fumble mechanics!

Fumble rules are hard to write, though: there are a lot of pitfalls.

• Fumbles should introduce complications, not punishments: no permanently-broken weapons or missing the next turn. Fumbles should add player energy, not suck it out.
• Fumbles should not render all characters incompetent boobs. One of Rory’s 3e DMs ruled that on every natural 1 on an attack roll, the character made an attack on an ally. Rory was playing a high-level ranger with many attacks per round. That meant that once every few rounds – several times a minute – Rory accidentally shot an ally. Not very Aragorn.
• Fumbles should be player-directed. Right now, players tend to exercise a little narrative creativity when they roll a 1. This is nice.

The natural-1 rules from the 4e Darksun books are pretty good fumble rules. They actually give characters a mechanical benefit – rerolling an attack – in exchange for a cinematic failure – breaking a weapon.

D&D design principle: Natural 1 is magic. Every d20 roll should have consequences for rolling a natural 1: consequences in addition to normal failure.

Do you guys use any cool fumble or crit effects for initiative rolls and skill checks? (Attack rolls and death saves already have special effects on a natural 20.)