Surprise!

I'm in the process of running an AD&D adventure from an ancient edition of Dragon (#63, to be precise), minimally converted to fit my bastardized Swords & Wizardry variant. It's been very useful in revealing to me just which game-mechanics tinkering is actually worth the trouble, and which can be discarded.

I've been using the very basic surprise mechanic that D&D and AD&D started with: 1d6, with most characters or critters being surprised if they roll a 1 or 2 (roughly 33% chance of surprise). It's simple, which is good, but it lacks granularity: at best, assuming your character can be surprised only on a 1, the best possible chance to be surprised is still about 17% — pretty high really. It doesn't really accommodate the preternatural alertness of fantasy tropes like Conanoid barbarians or Tolkienoid elves.

I like the idea of a single die-roll though, without having to mess about with modifiers and what-not, and I thought that since we have all these funny-shaped dice, why not make use of them?

To that end, I decree that the actual die rolled for surprise will depend on the character class, species, and possibly level. The default die remains the d6.

• Naturally alert critters like elves, barbarians, rangers, watch-dogs etc. are only surprised if they roll a 1 (16.6%).
• Very alert critters like ettins, mid-level barbarians or rangers etc. get to roll a d8 for surprise instead of a d6, still requiring a 1 to be surprised (12.5%).
• Extremely alert critters or mid-high level barbarians/rangers get to roll a d10 (10%)

And so on, up to a d30 (if you own one of those things, which I don't).

Things like wearing a vision- or hearing-restricting helm would drop your surprise die down one or two levels, so a fighter wearing a closed armet would be rolling a d3 for surprise (66.6%)

Now, all of this palaver could be sorted out easily by converting the surprise mechanic to a d20 or d% roll, but that has at least three strikes against it:

1. It makes it more difficult to adjudicate the effects of monster surprise descriptions, like those that surprise on a 1-5, and
2. It makes the "number of segments of surprise" thing more difficult to work out, and
3. It doesn't use the whole range of funny-shaped dice.

At the moment this is all theoretical, and hasn't been tested in actual play, but I think it could be a goer.