So one of my internet friends wrote up a system because he was tired of adding house rules to B/X but still wanted something that could run "old school content." I've played in it a whole bunch and I think it's pretty neat. In fact, right now, I'm working on some stuff I intend to run in the system.
This is the Surprise Table, as of revision eight, at least.
The procedure can be modified by Skills that the characters might possess, Alertness and Stealth, but we're going to first focus on the basic table. One of the most important things to note about the table is that there is a 50% chance of the players being surprised and the monsters being surprised, and these chances are independent of each other. The system in B/X also works this way, it's just a little clunkier. Both sides roll a D6 and if it rolls a 1 or 2, that side is surprised.
In lots of OSR systems and bestiaries, some monsters have decreased chances of being surprised or increased chances of surprising the players. One of the classic examples is the B/X bugbear with its 3-in-6 chance of surprising the party. There's also the less classic cloud giant who is only surprised on a 1-in-6 due to their keen senses of sight and smell.
The first thing to notice about the table is that even though there's a 50% chance of monster surprise and player surprise, the distribution of the results is not even. It goes 4-6-6-4 instead of the 5-5-5-5 one might expect. According to the author, mellonbread, this was to increase the likelihood of a meeting engagement and decrease the chance of one side surprising the other and annihilating them. This has the side effect of lowering the degree of modifiers you can add to the roll to represent conditions of concealment or awareness. In the even system, you can add or subtract up to 4 from the roll without completely eliminating one of the results, in the uneven one, you can only add or subtract up to 3.
So lets say you're running a module in Begone, FOE! and you come across one of these creatures and want to able to model its stealthiness or awareness. Your first thought might be to add a modifier to the roll. If the monster is stealthy, you could roll D20-1 or even D20-3. However, while this increases the chance that the players will be surprised, it also decreases the chance that the monsters will be surprised. That works fine if they're stealthy because they've laid an ambush and are watching for prey, or the players are making a racket, making it hard to hear approaching random encounters. But it doesn't work if the creatures have soft feet and quiet footsteps but are just as unaware of their surroundings as the PCs.
But there is another way of making the monsters stealthy or alert: treating certain results as if they were a different range of results. This was something I came up with in a fit of inspiration in the middle of the night. As an example, changing results of 11-16 to be 5-10 makes players 80% likely to be surprised, but the monsters still have a 50% chance of being surprised. This has the side effect of making it so someone is always surprised. For slightly less stealthy monsters, we can shift results of 17-20 to be 5-10, resulting in a 70% PC surprise rate. Though, this makes it impossible for the PCs to catch the monsters by surprise. If the monsters are surprised, so are the PCs.
Likewise, for alert monsters, we can shift results of 17-20 to be 11-16. That means monsters are only surprised 30% of the time. This also means that the players can never catch the monsters by surprise, but at least this time it makes sense. For slightly more alert monsters, we can shift results of 5-10 to be 1-4, resulting in the monsters only being surprised 20% of the time. This makes it so the monsters are more likely to catch the players by surprise.
But does this work with the character Skills of Alertness and Stealth? Alertness lets the players reroll any roll of 10 or lower and take the new result if it's higher. Meanwhile, Stealth grants the players the ability to roll twice on the table and take the highest roll. First, we need to look at how the skills transform the spectrum of results.
With Alertness, there is only a 25% chance the players will be surprised, and only a 4% chance that the monsters will be able to spring an ambush. Monsters are still just as likely to be surprised. Technically the monsters have a 51% chance of being surprised but we can probably ignore a single percentage point.
Stealth produces...interesting results. Like Alertness, there is only a 25% chance that the players will be surprised. However, there is only a 57% chance that the monsters are surprised. I wasn't expecting to get this result, and honestly, I'm still having trouble wrapping my monkey brain around it. The skill's name and purpose in the rest of the system primed me to think it would radically increase the chance of monster surprise, but it doesn't.
Going back to Alertness, we need to see how it interacts with our category shifting monster rules. Slightly stealthy monsters have a 55% chance of surprising Alert players (down from 70%) and a 51% chance of being surprised themselves. Stealthier monsters have a 70% chance of surprising Alert players (down from 80%) and a 96% chance of being surprised themselves. That's...another result I didn't expect. From this, we know we either have to scrap one of the "category shifts", or alter the original surprise rules. After all, having Stealth doesn't make you that much more likely to surprise your enemies.
Ideally, Alertness shouldn't alter the behavior of more alert monsters, but something tells me that's not going to be the case. Mildly alert monsters have a 21% chance of being surprised by Alert players, bizarrely down from a 30% chance, as if the players' heightened awareness was a result of echolocation that the monsters could hear with their finely tuned ears. Alert players still only have a 25% chance of being surprised (thank god). More alert monsters have a 30% chance of being surprised by Alert players, up from the 20% chance they used to have. I can't even think of a diagetic reason for that. Fortunately, we again have an unchanged chance of Alert players being surprised by Alert monsters, 25%.
I don't have the mental stamina to see how Stealth interacts with stealthy and alert monsters.
From all of this I can conclude that category shifting does not work with Begone, FOE!'s surprise rules. However, I hope that at least the first part of the post was helpful in understanding the probabilities of the original system. I may come back to calculate the results of Stealth, but more likely, I'll try to think of something that can transfer over B/X
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